Carlsen says "Det er i matt i neste" (true story)
What do you say one move from mating your opponent!

I usually don't say anything. I mean if it's offline, it's the usual good game/etc. I disable chat for various reasons on livechess specifically, this being one of them.
Though there have been some exceptions here.
Well, it depends. If there's more than one way to mate in 1, I tend to say in a jokingly way "Choose your poison". Other than that, I don't usually say anything unless the opponent has already talked to me during the game.
But if they tell me they're going to checkmate me in 1, I still play it out. I actually make my opponent waste 10 seconds of their life actually delivering the checkmate.
Once I played this person who was winning because I blundered terribly and dropped my Queen. But I still played it out through the endgame and he dropped the Rook, and he resigned immediately after I replied. Funny fact is, I never saw he dropped the rook, but he still resigned. So I asked him after the game "You won the match, why did you resign?" And he said "I dropped my Rook" and I finally saw it and I said "but I didn't grab it" and he said "Dropping a Rook is worth the defeat. I didn't deserve to win for doing that". It seems like the guy is pretty honorable. I don't know.

I go
"How do you feel?'
"Do you feel ashamed dat you lost "
"If you don't, well, then, lucky you."
"BUT YOU SHOULD"
BlARHGHLHA BLLAVHLADHBLALR LBALLLRLBALLRL RA LBLLALR
king me