which

"'A Date Which Will Live in Infamy'"

"Which?" is used by some English speakers, e.g. on Dublin's north side, when they meant to say "Sorry, I didn't understand what you just said."
Gilbert: Please hold your orthorexic lessons while I'm eating!
Michael: Which?

used to give more information about sth: ◆His best movie, which won several awards, was about the life of Gandhi. ◆ Your claim ought to succeed, in which case the damages will be substantial.
Jess Mastriani: Because it gives the impression that we have a rhythm together, that we have things in common, that we are on the same wave link! Which we are not!