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