A swedish expression meaning a good-looking person who is a teaser, in a really not-sexy-only-annoying way.
The word origins from boys trying to describe a hot girl (pussy), who literally leaves guys with their pants down, without finishing what they started, just because they think it's funny. Nix is slang for "nope" in swedish.
- Did you score with Monica yesterday?
- No, she was such a fucking nixcat.