A term used in boxing and other fight sports where a fighter, in order to compete in a lower weight division, will attempt to "drop weight", loose legitimate weight whether it be fat or muscle, and possibly "cut weight", loose weight by dehydration.
"Cutting weight" is considered by some to be less than honorable, since after the official weigh in, sometimes days before the actual fight, competitors will rehydrate themselves and end up at their real weight, which places them above the upper limit of that particular weight class.
The advantage of cutting weight is that once the fighter passes the weigh-in, he/she is free to regain that weight for the fight, as there are usually no further checks, and thus will be a heavier, more powerful fighter, in a lighter weight division.
The disadvantage to cutting weight and dropping weight in general is that if the fighter fails the weigh-in, he/she is disqualified and cannot compete. Also, a fighter may not be able to fully recover from dehydration by the time of the fight and will be weakened as a result.
Did you hear Little Mac dropped down to the 156 to 170lb weight division? He said he cut weight from 175lbs all the way down to 170lbs, but he should be back up to 175lbs for the fight. He's gonna tower over every other welterweight.