treppenwitz

Literally, 'the wisdom of the stairs'. The striking reply that crosses one's mind belatedly when already leaving, on the stairs. People are often angry because they did not have the fitting answer directly during a conversation. The term is old, but it was made popular by W. Lewis Hertslet who published his book in 1882 entitled 'Treppenwitz der Weltgeschichte'. In that book, he writes: 'Like to a petitioner who is just leaving after an audience, a piquant, striking words occurs to history almost always delayed.'

Yep, that's always when I think of the really witty zinger... half an hour too late.