A scrub is a more derogatory form of the slang "noob", "nub" etc. and is primarily used in video games, particularly those which are multiplayer based.
Whilst a noob is generally considered one who is new to the game, and as a result not proficient, a scrub need not necessarily be new. It may imply that the person accused is forever a noob, regardless of how much they may practice.
"FML why am i always on the scrub team"
"You piece of shit scrub, uninstall"