It can't really be defined. Nojob is a word you can use for alot of stuff. Like people and deeds. In theory it means that someone who is a nojob won't get a job, or at least only get a bad one. Like a construction worker who used to smoke crack and didn't pay attention in school.
Example 1
Guy1: Look, that guy is smoking weed in class!
Guy2: Hahhaha, what a Nojob!
Example 2
Guy1 and Guy2 talks to another random guy at the start of a new school year. The random guy tells them that he used all his savings on cigarettes and that he got an F in some of the classes last school year. Guy1 and Guy2 walks away and Guy1 whispers "WTF! That's super nojob hahaha!"