Originated from Jerome Bettis of the Pittsburgh Steelers, because back in the day the Steelers would do all the hard work on a drive down to the 1 yard line and then put Jerome Bettis in to pound in a touchdown and then reap the reward afterwards when all he did was use his weight to jam in a yard! It was ridiculous. So a Bettis is a term used in the following ways:
1) Someone does all the dirty work and then you come in and finish it up to claim all the responsibility
2) You do all the dirty work and then someone else comes in and claims all of the responsibility.
This term was developed by my roommates and I while playing the game Halo 3. Someone would just abut kill someone and then someone else got the kill instead. Below are some examples of how this term is used.
1) Oh what the hell was that! You bettised me man! What a bettis!