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late 13c., "buttocks, anus," from Old French fondement "foundation, bottom; anus" (12c.), from Latin fundamentum "a foundation," from fundare "to found" (see bottom). So called because it is where one sits.
Alle þe filþ of his magh ['maw'] salle breste out atte his fondament for drede. ["Cursor Mundi," early 14c.]
fundament fun·da·ment (fŭn'də-mənt)