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"