A slimeball is a mean and dishonest person.
Calling someone a slimeball is a very strong insult.
Examples:
He is such a slimeball, always lying and being mean.
I don't want to be friends with a slimeball like him.
The slimeball tried to steal my lunch money.
The slimeball thought he could trick me, but I'm too smart for that!
That slimeball is the worst person I know.
Don't be a slimeball, be nice to people instead.