Lemon
Basically, to describe a person(s) who is or is known do be a dumb arse or that does dumb shit. Originated from 'Slot Machines' and spinning up Lemons = a shit spin/result etc.
Example 1.
Mate 1 : "Dude, Damo got so drunk last night he spewed all over the floor"

Mate 2: "What a fucking Lemon"

Example 2.

Driving in your car and getting cut off - "Learn to drive you fucking lemon!"