mammoth jack

American  

[mam-uhth jak] / ˈmæm əθ ˈdʒæk /

noun

1. Informal. a notably large male donkey.

2. any of various large donkey breeds, including the American mammoth donkey, a landrace bred for strength from several donkey breeds brought into America in the 18th century.

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Etymology

Origin of mammoth jack

First recorded in 1835–40; jack ¹ ( def. ) (in the sense “male donkey, jackass”)