afterwit

Wisdom which comes after the event.

After wits are dearely bought, / Let thy fore-wit guide thy thought.

1692, Roger L’Estrange (translator), Fables of Æsop, London: R. Sare et al., Fable 162, “A Nightingale and a Bat,” There’s No Recalling of what’s Gone and Past; so that After-Wit comes too Late when the Mischief is Done.

The lack of forethought.

A good comeback, retort one thinks of only after the end of discussion or after leaving a social gathering.