Slimy and unpleasant people who cannot be trusted.
Slimeballs are very bad and no one likes them.
Examples:
I don't want to be friends with slimeballs.
He is such a slimeball, always lying and cheating.
The slimeball tried to steal my lunch money.
The slimeballs are always up to no good.
I've never met a bigger group of slimeballs in my life.
The slimeball's smile was as fake as a plastic flower.