Firstly, the name 'rock': The phrase 'rock and roll' had sexual connotations long before it was the name of a genre. It's a pretty good term for things sexual, because when things get heated, beds do rock. And a certain amount of rolling around occurs. Nick Tosches' 1977 book Country: The Twisted Roots Of Rock And Roll mentions the phrase being used in a 19th century sea shanty, while there's a 1922 Trixie Smith blues song called "My Daddy Rocks Me (With One Steady Roll)".

The term 'rock and roll' was first used to name the genre of upbeat dance music by the 1950s radio DJ Alan Freed. Freed famously played black 'rhythm and blues' music to white teenagers, and he seems to have started calling that music 'rock and roll' on his radio show in 1954. For Freed, the name 'rock and roll' removed the racial connotations of 'rhythm and blues' or 'race music'; after all, it a decade before the civil rights act of 1964, and Freed was trying to appeal to white teenagers.

Of course, 'rock and roll' is actually a different genre to 'rock'. The term 'rock' now usually specifically denotes a genre of music that rose to prominence after 1965-1966, which had very different motivations to 'rock and roll' despite having roots in the genre. That is, in 1965-1966, a genre of music with rock'n'roll roots, countercultural aspirations, and aspirations towards art became very prominent; the Beatles who made fun rock'n'roll in 1963 had by 1965-66 started experimenting with previously quite avant-garde studio techniques and songwriting styles. Albums like Bob Dylan's Highway 61 Revisited, The Beatles' Sgt Peppers, & The Mothers Of Invention's Freak Out aspire to art in a way that Little Richard did not (note that aspiration to art is a separate story to the artistic worth of the music, because many see artistic worth in 'Tutti Frutti' despite Richard Penniman's intentions). That stuff is 'rock' rather than 'rock and roll'.

After all, 1965 also saw a popular band (The Rolling Stones), a popular song ('Like A Rolling Stone' by Dylan), and an increasingly influential magazine (Rolling Stone) all of which focused on this new 'rock aesthetic'. And each of these mentioned small rocks (i.e., stones) in relation to rolling. In this era the connotations of 'rock' clearly became less about the sexual thing and more about...mineral conglomerates originally found in the ground. After all, there's a link between sex and dance music, and as the music became less about dance music, the original meaning of 'rock and roll'- sex - lost ground to the mineral conglomerate meaning.

Let's move across to the name 'metal': luckily for us, there's a very detailed 2014 article by Deena Weinstein in Rock Music Studies discussing the origins of the term 'heavy metal', which I am totally cribbing from. Weinstein argues, largely, that 'heavy metal' was a genre name that was waiting to happen. There's just a lot of different cultural references pointing in that direction, and it makes semiotic sense as a descriptor of the music.

Firstly, louder 'rock' music with distorted guitars - Cream, Jimi Hendrix, The Who - often got called 'heavy rock' in 1967-1969. As the music got louder and more distorted, critics searched for terms to describe the extremity of the volume and distortion that felt more effective than 'heavy rock'. In a 1968 review in Rolling Stone of Hendrix's Axis: Bold As Love, Jim Miller described the music as "like a junk heap (Ben Calder crushed monolithic mobiles bulldozed), very heavy and metallic loud". Lester Bangs' early reviews also used 'metal' as an adjective referring to volume and distortion.

Secondly, a series of William Burroughs books in the early-to-mid-1960s has a recurring character called Uranian Willy The Heavy Metal Kid, and this might have been an influence on some of the early users of the term 'heavy metal'. In particular, the older roadies who worked for the heavy rock band Blue Cheer, who were fans of Burroughs, apparently nicknamed the group 'the heavy metal kids'. Steppenwolf's 1968 track 'Born To Be Wild' features the line 'heavy metal thunder', which might also be inspired by Burroughs (they are a band named after a Herman Hesse book, after all). The critic Lester Bangs definitely aspired to Burroughs' beatnik prose.

Finally, many of the bands described as 'metal' - Black Sabbath, for instance - came from big industrial areas like Birmingham in the UK or Detroit in the US, which presumably featured steelworks and the like. The title of one of the biggest albums by one of the mid-70s bands who were quite early in consciously thinking of themselves as metal - Judas Priest's 1980 British Steel - makes this link quite explicit.

Anyway, the first person in print to put the words 'heavy' and 'metal' next to each other in order to describe music that features loud guitars and headbanging (etc) was probably either Lester Bangs or Mike Saunders. In a February 1970 review of a Guess Who album, Bangs describes them as being "quite refreshing in the wake of all the heavy metal robots of the year past", but you could also argue that it's slightly ambiguous whether Bangs is describing them as 'heavy metal robots' or 'heavy metal robots' - Bangs was the kind of impressionistic writer who really might have been saying that the band were metal robots. In a November 1970 review of three Humble Pie albums, Mike Saunders refers to one Humble Pie album as being "more of the same 27th-rate heavy metal crap", which is more obviously a genre description. In an interview with Weinstein, Saunders says that his use of the phrase 'heavy metal' was by analogy to 'heavy rock', imagining 'heavy metal' would be heavier than 'heavy rock'.

However, just because Saunders and Bangs were calling the music 'heavy metal' in reviews doesn't mean it quickly became the agreed-upon term for the music. In a 1971 Rolling Stone profile of Black Sabbath - who are often called the first heavy metal band these days - their musical style is called 'downer rock'. A 1972 Lester Bangs thinkpiece in Creem about Black Sabbath's style of music also uses the term 'downer rock'. In general it's only in the mid-to-late 1970s, with the rise of the New Wave of British Heavy Metal bands like Judas Priest and Iron Maiden, that the descriptor 'heavy metal' becomes the most common way to describe that music.