a) when something sounds too good to be true
b) when someone is way too optimistic about the outcome of a certain decision
They offered you a job starting at 80k and they also pay for your MBA? That's gotta be impositive!
dude 1: "You sure you can win this poker game?"
dude 2: "Sure? I'm gonna set a new record, dude!"
dude 1: "Man, you are impositive!"