The foundation or base of something; a fundamental principle or core truth; sometimes used euphemistically to refer to the buttocks.
From Latin fundamentum (foundation), from fundus (bottom). The word entered English in the 14th century meaning 'foundation' and later took on euphemistic bodily meaning.
Medieval and Renaissance writers loved 'fundament' for its ability to sound scholarly and prudish while actually discussing something amusing—it's a delightful example of linguistic displacement in action.
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