I have heard this term being used to describe a particular Pokemon in a person's team. It comes out of the competitive scene, although I don't really get what it means.
For example, a blog post about Flygon on Smogon which describes it like so:
Flygon is a staple on many teams as a scout, revenge killer, and all-around reliable clutch Pokémon.
I don't think the term is necessarily restricted to Pokemon as I can recall hearing it in other games such as Counterstrike and Call of Duty.
What does it mean?