Definition of stack off
stack off means taking all the stack from an opponent. In other words, taking all of a player's stack in one move. If you went in against a player who has the same stack as you and you win the jackpot (and in turn, his stack), it will be said that you have de-stack ed that player. It can also be called a "knock-out" against your opponent. For example, you have AA and your opponent has KK on A-A-K-K-2, so chances are your opponent will make stack off.
Use of the term stack off
As a general rule, in games of cash games in which money is played, we will more often talk about cellars, so if a player loses all his money, we will use the expression "he's been stripped" more often. The expression stack off will be used more often in tournaments in which you build an stack more than a mat. It's all a matter of convention, but you'll hear stack more often in tournaments and mats in cash games.