Definition of spew
The fact of spew is wasting your chips or money. A player can spew for several reasons. He can spew because he is tilted. Often, after a big badbeat, you will see a player push all in his next 8-9 hands. If he loses several buy-ins as a result, he will be called spew. We can also spew because we don't play our A-game. If we decide to bluff a calling station on which we've marked "don't fold", we'll call spew. The spews can also be more subtle. If you call an all in with QQ against a player who only pushes all in with AA, we'll call it a spew.
The different reasons for spew
As noted above, some situations are obvious spews and others are more subtle spews. We can spew of the stacks because we are tilt. We can spew because we are tired. And we can also spew because we play too low. Indeed, if you're an NL1000 player playing NL5, you probably won't have much consideration for the size of the buy in, and therefore, you might do spews.