don ‧ key /ˈdɒŋki $ ˈdɑːŋki/ BrE AmE noun
[ Date: 1700-1800 ; Origin: Perhaps from dun (=gray-brown) + -key (as in monkey ) ]
1 . [countable] a grey or brown animal like a horse, but smaller and with long ears
2 . donkey’s years British English spoken a very long time
for donkey’s years
I’ve had this jacket for donkey’s years.