@ommmm Ok, in that case, it just means "yes". We use it to say "yes" when there are negative questions. Of course, the meaning can also depend of the intonation. That could be obviously, ironic, sarcastic, etc.
I think I see your point. You mean "si" as an answer to a negative question. That's why "obviously" (évidemment, bien sûr) is probably not good choice as "yes" in English simply works. Look at the following to have a feel for reason why "si" exists in French.
Sample: Wont you come? Tu ne veux pas venir ? Yes! Si ! => I will come No! non ! => I don't wanna come Obviously! bien sûr ! oui ! => quite undefined what is meant, come, not come... even the best guess is you don't want to come, a little bit ambiguous though, a reason why we have "si" in French ("doch" in German).
Does it make sense? Back to your "obviously", and referring to my example, you should decorate it like "bien sûr que je veux venir / ne viendrai pas...".