A buttock or rear end (informal and slightly vulgar); can also mean an elephant's tusk.
From Yiddish 'tush' or 'toches' (buttocks), from Hebrew 'tachath' (under, beneath). This is a relatively recent addition to English slang from Yiddish speakers in 19th-century America, replacing older Anglo-Saxon words.
Tush is a great example of how English borrowed informal body-part words from Yiddish—a sign of cultural mixing in American cities. It's less vulgar than older words, which is probably why it stuck around!
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