In the popular multiplayer shooter Team Fortress 2(tm) made by Valve(tm), spies are a nuisance. They run around, backstabs players and disguises as players. But above all else, they are SAPPIN' MAH
SENTRY!
An engineer builds stuff. A spy saps the stuff
the engineer builds, and utterly destroys it in a matter of seconds. Naturally, they are really annoying.
The engineer thinks so as well. That's why he yells out "
Spy's sappin' mah
sentry" in pure fear and helplessness.
Also the inspiration to "
spy's a spy!", created using digital soundboards.
The phrase is mostly used to describe a sense of fear, despair or pure hate when someone completely ruins whatever you're trying to accomplish.