1. A term used by an individual who has not quite made up their mind on a question they have been asked.
2. A response given by an individual who is asked to do something with another individual. This response usually indicates that the person is unwilling or unable to confirm their participation with the other individual, or just doesn't want to participate and doesn't like telling people "no thanks".
Person A: "Hey Mac, want to go to the movies tonight?"
Person B: "I'm not really sure what I'm doing tonight, but I'll see what's up!"
Person A: "Hey Mac, we are going to the bar tonight. Want to join?"
Person B: "Yea, I'll see what's up..."