This is usually a term for someone who is new in school or a sport team. Most often a freshman or a transfer student. Typically used by someone who was butt-hurt when they were a scrub the year before.
Sometimes there is one said "scrub" that tries to be popular and attempts to make "scrub" a good name, by calling himself the King Scrub or Scrub
Ultimatum. Usually this backfires when the
upperclassmen call him that name and he gets made fun of even more.
Equivalent of "noob", "newbie", or "loser"