Abbreviation of Hoist By Your Own Petard.
Could also be "HBHOP" for "His Own".
Part of the Counter-Strike "culture". Used by pedantic geeks to make fun of people that do damage or kill themselves in game.
"OMG, lolers! U GOT HBYOP!"
"For tis the sport to have the enginer Hoist with his owne petar". - Shakespeare, who gives the line to Hamlet