When playing a co-op game, a Backstab Murphy is when one player desperately needs a powerup of some sort (usually health), only to be denied the powerup by his fellow gamer who they happen to be co-oping with.
Player 1: Oh, man. A health pack! Finally!
Player 2: SWIPE!!
Player 1: ... Did you seriously decide to go Backstab Murphy on me?
Player 2: SWIPE!!
Player 1: ... Did you seriously decide to go Backstab Murphy on me?
by Nyxandria December 7, 2009
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