Well, this time I'll actually post it, albeit a little late. II'm sure it's still Sunday SOMEWHERE!
L'esprit d'escalier (L'esprit de l'escalier)
Literally staircase wit - thinking of a clever remark or retort when it is too latye or, when you are literally walking down the staircase and it is too late to be useful.
From French esprit (noun), meaning humour or wit + escalier, stairs. The name was coined by French philosopher Denis Diderot's description of the situation in Paradoxe sur le comédien, where he states that at a dinner a comment was made that left him speechless, at which he explains:
l’homme sensible, comme moi, tout entier à ce qu’on lui objecte, perd la tête et ne se retrouve qu’au bas de l’escalier. [A sensible man like me, overwhelmed by the argument levelled against him, becomes confused and can only think clearly again when he reaches the bottom of the stairs.]
1906 H. W. & F. G. Fowler King's Eng. i. 32: No one will know what spirit of the staircase is who is not already familiar with esprit d'escalier.
Being surrounded by a lot of people at work all the time, I often find myself needing to come up with some kind of witty retort quite regularly. I usually try to avoid argumentm ad absurdum or ad hominem, but I do find that if nothing else comes to mind, I can avoid a case of l'esprit d'escalier by just resorting to the old, ever-faithful "That's what SHE said!" or "And that's what your mum said last night!"