A bastardized misspelling of the English word "please", usually spoken by Polish people on the internet, particularily in games such as Enemy Territory and World of Warcraft. Sadly this is just one of many English words the Polish have bastardized, including "give" (which many Poles spell as "gief").
An example from World of Warcraft:
"Trade Polish dude
: PLIS GIEF 1G FOR TABARD!!!!!! PLIS!"
An example from Enemy Territory:
"PuszcadfPL: PLIS KICK CHEATER AIMBOT NOOOOOOOOOB!!!!!!! KURWA"