Scrub is generally used as an insult for someone who sucks at a video game or sucks at doing anything at all. The meaning being basically the same as "noob"
A scrub is any person with the name of Zac, Zach, or Zack and can also be called by the name of Zscrubs.
This person cannot handle anything at all especially video games.
Zach is the biggest scrub I know especially when it comes to playing Playstation.
A scrub is someone who is so god damn stupid that they drop the IQ of everyone they look at. They usually make terrible puns and really lame, overused inside jokes that nobody thinks are funny. They have high, ear-piercing voices that shatter ear drums. They have no friends and they are generally found camping on Call of Duty with a K/D of 0.1 or on Minecraft building random, ugly-ass shit.
Scrub: "OMG THIS GAME IS RIGGED! WHAT THE HELL!!!"
Person: "JESUS CHRIST, KILL YOURSELF, SCRUB. YOU'RE STRAIGHT GARBAGE! GO DRINK SOME BLEACH!
Your best friends husband that will get her pregnant and won't man up and take responsibility. Spends his day at home doing nothing while she is working all day to pay the bills and rent.