Vicious pair in shower? (3-3-3)

I believe the answer is:

cat-and-dog

I'm a little stuck... Click here to teach me more about this clue!

'vicious' is the definition.
Both the definition and answer are adjectives.
Maybe you can see a link between them that I don't see?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Another definition for cat-and-dog that I've seen is " See 7".)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.