"JIT" is not always used as an insult, despite the fact that it can seem condescending.  In jest, friends may refer to one another as "JIT," particularly when making fun of someone who is younger or less seasoned.  In other contexts, it could seem more patronizing, particularly if it comes from an older person or someone in a position of power.  Like many slang phrases, the meaning of the word is important.  Determining if "JIT" is intended as a joke or as a sign of disrespect requires understanding the speaker's tone, context, and relationship to the listener. Know more about what is a jit