1895-1900; probably orig. donkey('s)ears, as rhyming slang for years, with years replacing ears once rhyming orig. was forgotten
British Dictionary definitions for donkey's years
donkey's years
noun
1.
(informal) a long time
Slang definitions & phrases for donkey's years
donkey's years
noun phrase
A very long time •Chiefly British use: I got interested in his apprenticeship donkey's years ago
[1917+; a punning allusion to the long ears of a donkey]
Idioms and Phrases with donkey's years
donkey's years
A long time, as in I haven't seen her in donkey's years. This expression punningly alludes to the considerable length of the animal's ears. [ Early 1900s ]