1. A filler word used when you're either too lazy or don't care enough to respond to someone with a sentence.
2. A replacement word for "hello" or "hows it going."
3. When used inquisitively (e.g. with a question mark after it) is a replacement for something baffling.
4. A replacement for the word fuck
1. Person 1: Did you clean your room?
Person 2: Cluck!
Person 1: Why didn't you?
Person 2: Cluck you!
Person 1: Do you even get what I'm saying?
Person 2: Cluck?