Someone who doesn't care much about what they do or how they look doing it. Or if you just do something very very stupid.
((Girl walks into classroom for a class one day dressed in XXL sweatpants and a huge baggy sweatshirt, no makeup, hair looks nasty, but shes in perfect good health, just very lazy))
"Wow Anne, you are dressed like a scrub."
(Someone just hurdled a ping pong table to impress a girl, and crackled their head open, needing 5 staples in their head.)
"That was a really scrub thing to do"