Ah, Ngram--or rather, the OCR on which it is based--has deceived you.

Google's software for reading digitized older texts isn't perfect, thanks to different typefaces, page layouts, and other factors. I ran a Google Books search for "fart" and narrowed the date range to 1650-1700. While there are some genuine results, a large proportion of them appear to be a misread of "part", including in abbreviation (and in Latin, which probably helps explain the concentration in the 17th century as opposed to the 18th).

It's too bad. "The covetous fart of sacrilege" is a much better turn of phrase than "the covetous part of sacrilege."