Spirit of the Staircase
The French have this phrase, l'esprit d'escalier. Literally, it translates as "the spirit of the staircase" but what it actually means is the things you think of to say after it’s already too late and you’re on your way out.
The spirit of the staircase was with me the whole walk home as I thought of better comebacks I should have said to him when we had that argument.