Once there, Willow says other cryptic words "ah, l'esprit d'escolier". In French "l'esprit" means spirit and "escolier" is a weird hydrid of the words "ecolier" - "pupil" and "escalier" - "staircase". (I wonder if it's a clue to some future plot twist or just a misspelling).

It's probably a misspelling beacause "esprit d'escalier" actually means something in French!

That or Willow has studied old French when "écolier" was still "escolier".

Oh Escolier est aussi le nom d'un poisson, c'est le Gempylus serpens !