An excuse given when running late to work, wedding, party, etc. Instead of trying to make up something, you just say you hit a cat, that is why you're late. Hitting a cat wouldn't impact anything, in terms of time, so it's not an excuse, but more or less done to add amusement to the tardiness and delfect any heat you would get otherwise.
Text Message: Can we move our meeting till later in the day, I hit a cat on the way there.
Your boss: Lol, I was about to fire you, but this is clever, I'm going to reuse this.