Tsitaat Algselt postitas nikitheone
A donk bet is a bet where, on the previous street, the player called either a bet or a raise. On the current street an apparent blank falls and the player now leads out into the bettor/raiser.

Although the term is deragotory it can often be correct play (as has been mentioned). It's also a classic move by the donk that just hit his 2 outer to beat your TPTK.
i know that