A jokebag is a person who is a joke, full of shit, or simply someone you don't like, respect, or has traits of a doucher. Jokebags tend to be ignorant, clueless, and lucky too.
In poker a jokebag is someone who lucks out on you for playing shitty cards (also known as a luckbox). Jokebag can pretty much be used in many different reasons and context, thus making it the greatest word of all time.
Synonyms: Luckbox, n00b, Scrub
You see that guy suck out with that two-outer? What a jokebag!
Man what a douche, how is that girl with that jokebag?