Interesting term for a woman’s breasts. Not inapt actually when one thinks of the cleavage and of builder’s bum, as it’s known in the UK (or the crack of doom as it’s known in our house). OED labels it obsolete and jocular. Rare too I fancy as I’ve only come across it once or twice in my reading.
Incidentally the cite OED gives for it may tend to mislead. Here it is:
† fore-buttock n. Obs. (jocular) the breast (of a woman).
1727 Swift Sylv. in Misc. IV. 137 Her Fore Buttocks to the navel bare.
The unwary might come away from that believing the line is by Swift. In truth the Miscellanies was a compilation put together by Swift and his friends Pope, Arbuthnot and Gay, all of whom contributed pieces to the collection. The line in question is from Sylvia: A Fragment,which is by Pope. I excerpt:
Now with a modest Matron’s careful Air
Now her Fore Buttocks to the Navel bare
Many of the lines from the fragment Pope worked into his Moral Essays IV: On the Character of Women but the line in question he used again on one of his Horatian satires (Second Satire of the First Book of Horace), the place I first came across it
Nothing in Nature is so lewd as Peg,
Yet for the world she would not show her leg,
While bashful Jenny, e’en at Morning-prayer,
Spreads her Fore-buttocks to the Navel bare.
To the navel? Now that’s cleavage! (Or poetic licence, one of the two).