The OED, the premier source of English etymologies, says the meaning “female genitals” came first and the meaning of “backside” came second. Both are listed as “etymology unknown”. The older is labeled “Chiefly British English” and the younger is labeled “orig. and chiefly U.S.,” but, as we shall see, I’m not sure that’s entirely true.

The “female genital” usage first appears in print in the infamous magazine the Pearl in 1879, . /u/skedaddle, a historian of British and American humor, has an excellent post on “What sort of humor would be unacceptable to a 19th century audience? Do we have any edgy jokes from that period?” that mentions the the Pearl, calling it outright “pornographic”. The first two entries for this meaning of “fanny” are late Victorian:

1879 Pearl I. 82 You shan't look at my fanny for nothing.

1889 A. Barrère & C. G. Leland Dict. Slang I. 354/2 Fanny (common), the fem. pud.

The next citation is none other than the great James Joyce’s Finnegan’s Wake, where Joyce writes:

Two lads in scoutsch breeches went through her..before she had a hint of a hair at her fanny to hide or a bossom to tempt a birch canoedler.

It was no doubt in common usage throughout this period though, since it was obscene, it may well have not ended up in print.

The first known record in print of the backside meaning of the term is from 1928 Broadway play The Front Page, which includes the line, “Parking her fanny in here.” It seems, in this context, the meaning might be rude but not obscene. Other citations are similar from this period are similar. We have one from another play, 1930’s Private Lives, where one character says, “You'd fallen on your fanny a few moments before”. This play was interestingly a British play by a British playwright—Lawrence Olivier was in the original cast production. The next OED citation is from 1937, again from a play, and again a British one. This time the quote is a bit of dialogue, “That's it. Progress.” “Progress my fanny.” Neither of these uses unambiguously means “backside”. The next entry we have for fanny as backside is from a British-educated South African poet, Roy Campbell, and here it pretty clearly means backside:

But here you cannot crash a bogus entry Although the blitz was weathered by your Granny, Ere you came back to serenade the sentry, Who thanks you with this bayonet in your fanny.

In the same year, British poet Ezra Pound published his Canto LXXX, which featured the lines:

And three small boys on three bicycles Smacked her young fanny in passing.

Then in 1953, we have again a British author, Richard Gordon, using it in a novel, where one character says, “Move over, Second, and let the Doctor park his fanny.” Again, rude but not obscene. 1958, we have British pulp author saying, “In 1960, we have British-Australian author say “I'd never be able to think of John and Jo again if we just sat tight on our fannies and did nothing.”

Again, unambiguously British and unambiguously backside.

So this meaning was first recorded in America, and seems to have been particularly common among playwrights and poets around the English speaking world for a period and seems to have been somewhat common until at least the 1960’s.

Since it’s unclear where either of these meanings come from, it’s unclear if their etymologies are related. It seems like this a bit of an ambiguous word for a period, though only one meaning was commonly recorded in print (and presumably only that meaning was used “in polite company”). Think, perhaps, of the word “hook up” today, which is semantically ambiguous and commonly used to meaning everything from the light “make out” to the more serious “had casual sex”. Words’ meanings can change through a process called “semantic drift”. Silly, for instance, once meant blessed (blessed—>innocent—>weak—>foolish—>our current usage). It seems concievable to me that fanny moves from the female genitals to ambiguously describing the general region. That’s about the only way it makes sense to me.

In America, overall usage seems somewhat rare but I have yet to find a usage meaning “female genitals”. There doesn’t seem to have ever been that ambiguity. In Britain, as both informal and formal censorship decreased (the former because of the Sexual Revolution of the 1960’s and the later after the failed prosecution of an unabridged version of Lady Chatterley’s Lover in 1960), the meaning seems to have narrowed back down to the original sense of only a woman’s genitals, likely as more people started using the older meaning more publicly it sort of pushed the newer meaning it of common usage.

Where did the term come from in the first place? I think the two share an etymology. The OED implies they do as well. Again, the OED says this etymology is unclear, but I can present four options:

1. Fanny is a diminitive form of the name Frances (or Francine). It is that it’s simply the applying of a woman’s Christian name to the genitals, just as names (Dick, Willie, Jimmy, John Thomas, etc) are applied to male genitals.

2. This nickname could come directly from the 1748 much banned erotic novel Fanny Hill, Memoirs of a Woman of Pleasure, apparently the “first pornographic novel”. Here, author John Cleland chose the name “Fanny Hill” as a translation of veneris mons, the mound of Venus (mons means both mound and hill). I believe this area today is more commonly called the mons pubis. Etymonline, for instance, suggests this as a possible etymology for fanny. To me seem like the most likely source of the term (/u/AnnalsPornographie, who I would defer to on all matters Fanny Hill, thinks the same, see below), though there’s no direct evidence connecting the two.

3. “Fanny Adams” was British sailor slang for tinned meat, and there’s a long history of similar meat or food terms being applied to the vagina. There’s absolutely no evidence connecting the two, and the first recording of Fanny Adams as slang comes a decade after the Pearl first published fanny as genitals.

4. It’s easy to imagine some connection with actual fans or fan shapes (“the fanny bits”), but this is speculative and seems unlikely. Still, stranger terms have historically ended up as slang for vagina and penis.

Again, I think the second explanation is the absolutely most likely (it’s exactly what the author meant when naming his character, even). From there, it seems likely there was some degree of semantic drift in England and America where fanny referred to the whole region or simply the buttocks of anyone (male or female). As censorship and self-censorship decreased after the 1960’s, common usage in Britain dropped back to its original genital meaning whereas America, apparently having never been familiar with the genital meaning, continued to use the word to mean buttocks.

The term “bumbag” first appeared in Britain in 1951. “Fanny belt” appears in 1963 and “Fanny pack” in 1971, both in America. Why the bum/fanny when the bag part is worn in front? It was originally designed for skiers rather than tourists, apparently, and, not being worried about foreign pickpockets, they wore it at the back. Through the 70’s, it seems to have more commonly used by skiers and rugged outdoorsmen and then in the motorcycle enthusiasts seems to have started using them in the 80’s, before they were widely adopted by embarrassing parents on vacation and thereby losing all cultural cachet.