be another kettle of fish

be another kettle of fish

To be entirely different in scope or description from someone or something that was just being discussed. A: "So you like your first two roommates, but what about your third one?" B: "William? Oh, he's another kettle of fish altogether." I suppose we should start looking at buying a house now that our baby is on the way, but that's another kettle of fish!
See also: another, fish, kettle, of