kettle of fish

1. A troublesomely awkward or embarrassing situation.
2. A matter to be reckoned with: Making money and keeping it are two quite different kettles 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]

The noun has one meaning:

Meaning #1: informal terms for a difficult situation
  Synonyms: fix, hole, jam, mess, muddle, pickle