Definition of "buttocks"

At leaſt five Hundred of theſe reforming Vultures are daily plundering our Pockets, and ranſacking our Houſes, leaving me ſometimes not one pair of Tractable Buttocks in my Vaulting-School to provide for my Family, or earn me ſo much as a Pudding for my next Sundays Dinner : [...]

1707, Thomas Brown, “Moll Quarles's Answer to Mother Creswell of Famous Memory”, in The Second Volume of the Works of Mr. Tho. Brown, containing Letters from the Dead to the Living both Serious and Comical, part three, page 184

‘Children crawled over each other like little grey worms in the gutters,’ he said. ‘The only red things about them were their buttocks and they were raw. Their faces looked as if snails had slimed on them and their mothers were like great sick beasts whose byres had never been cleared. […]’

1963, Margery Allingham, chapter 7, in The China Governess‎