One of the mysteries of the English language finally explained.
The buttocks; the anus.
Early 17th century; earliest use found in Randle Cotgrave (fl. 1587–?1630), lexicographer. From nock + -andro, of uncertain origin, but perhaps from ancient Greek ἀνδρ-, ἀνήρ man, in order to distinguish it from a woman's nock.
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