Definition of "fundament"

It [the Sphincter Ani] serves to purse up the Fundament, and so hinders the involuntary Evacuation of the Fæces.

1703, Thomas Gibson, The anatomy of humane bodies epitomized

ANOTHER defect that new-born infants are liable to is, to have their fundaments closed up; by which they can never evacuate the new excrements engendered by the milk they suck […]

1861, Aristotle (pseud.), Aristotle's Works: containing directions for midwives, and counsel and advice to child-bearing women with various useful remedies., page 119

Bathe the parts frequently with cold water, and, if there be much pain at stool, always squirt up the fundament, beforehand, with a syringe, half a teacupful of cold water.

1864, Alfred Fennings, Fennings' everybody's doctor; or, When ill, how to get well, page 9

I flinched when he touched my rosebud, but pretty soon I was fucking his mouth like it was Hector's fundament.

2008, Eric Summers, Ride Me Cowboy: Erotic Tales of the West, page 38