Heinie

wiktionary

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From a corruption of *hindie, for *behindie(“behind, rear end, hindward parts”), equivalent to hind +‎ -ie.

etymonline

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Heinie (n.)

also Heine, Hiney, 1904 as a typical name of a German man, North American slang, from pet form of common German masc. proper name Heinrich (see Henry). Brought to Europe in World War I by Canadian soldiers (British soldiers called the adversary Fritz).

heinie (n.)

also heiney, slang for "the buttocks," by 1930s, of uncertain origin; perhaps a contraction of hind end.