fundament

noun

1. Foundation.
2. The bottom; the buttocks or anus.

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

   “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 […]”

   “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.”

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

3. The underlying basis or principle for a theoretical or mathematical system.