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!

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!