Keywords are inclusive, so the more keywords you put in, the less lines you'll find.
You can specifically ask for a keyword to not exist by prefixing it with !.
For example, searching for "mouse !cheese milk" would find lines that contain both mouse and milk but not cheese.

Keywords:
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2 lines found
[060511 01:49] d0190, Peanut: alesk
[060513 21:27] f2e1e, Peanut: