osa katt

From osa (“smell”) +‎ katt (“cat”). Perhaps in reference to impending punishment with a cat-o'-nine-tails (niosvansad katt).

osa katt (present osar katt, preterite osade katt, supine osat katt, imperative osa katt)

1. (idiomatic) to start to look risky

   När lagets bästa spelare utvisades kände man att det osade katt

   When the team's best player was sent off, you could tell they were in trouble ("one felt it smelled of cat")

Also appears to be used in the sense of "to seem fishy" by some, though ligga en hund begraven and ana ugglor i mossen are more idiomatic.

• katt in Svensk ordbok (SO)
• katt in Svenska Akademiens ordbok (SAOB)