One who labors hard and lives meanly; a mean fellow.
2
One who is incompetent or unable to complete easy tasks.
What a scrub! Instead of washing the dishes she put the used food on her face!
3
A thicket or jungle, often specified by the name of the prevailing plant
oak scrub
4
(stock breeding) One of the common livestock of a region of no particular breed or not of pure breed, especially when inferior in size, etc. Often used to refer to male animals unsuited for breeding.