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"
Guy: Dude, you're such a scrub!
Other
Guy: Weren't you a scrub last year?
Guy: Yeah, but I was a cool scrub. And you aren't! Ha!