In my opinion, no, it's not an insult. It's a term used when you're referring to someone who has the odds stacked against them, but there is an underlying amount of support that is there because there are hopes that they will win. In the end, even if they don't win, they were regarded as being someone (or if it's a team, "something") that won the people over.

Hi Mohamed I dont think it is rude, because underdog is someone who is starting from lover possition.This term is mainly using in sport .The underdog has defeated the champion. For example. Martin

Thanks a lot Alison

It depends on the person and context. The term under dog is not directly related to life but rather a skill or event. An underdog is someone who would have little experience compared to someone else they are competing with or someone else doing the same thing, usually used in competition based situations like sports. It can also be used in many other places besides sports though like in the work place. Example if you're a new sales agent and you have to complete with having more sales than your coworkers who have been there longer than you would be the underdog.