A Greb is classed as an idiot or a silly person that would do something inappropriate or rude. This word should not be used offensively ,it’s a jokey slangword.
Greb is an adjective. It describes someone who is a waster or doesn't look after themselves. For example someone who takes lots of drugs, doesn't work and doesn't take on any responsibilities. (Basically just a crack head)
Tabby - "God Tracy is such a dosser. She does nothing!"
Lilly - "I know right, what a greb!"