This great malaphor is a blend of”wet blanket” and “cold fish”, both reflecting rather sour personalities, the former a depressing person who spoils others’ enjoyment, and the latter a person who does not seem very friendly and shows little emotion. Fish of course are always wet (unless grilled I suppose) which may have been in the speaker’s mind as well. For some reason a dead fish handshake also seems in play here. I personally am going to start using this expression as there are people out there who fit both categories. John Costello gets the kudos for this one, as he unintentionally uttered this malaphor when he was describing a person who is kind of a dour drip. I hope it wasn’t me!