kettle of fish
1. Also, a fine or pretty kettle of fish . An unpleasant or messy predicament, as in They haven't spoken in years, and they're assigned to adjoining seats-that's a fine kettle of fish . This term alludes to the Scottish riverside picnic called kettle of fish, where freshly caught salmon were boiled and eaten out of hand. [Early 1700s]
2. a different or another kettle of fish . A very different matter or issue, not necessarily a bad one. For example, They're paying for the meal? That's a different kettle of fish. [First half of 1900s]