Gank

From OLSEncyclopedia

To gank something implies doing something sneakily for your own gain, often against the will or intent of another party. The term is common in MMORPGs like Guild Wars, where in guild battles the enemy's base has two entrances, making the enemy Guild Lord susceptible to being ganked while the enemy team is caught up fighting elsewhere on the map.

A successful gank usually means the other party just got pwned.