The phrase comes from Brazilian Portuguese slang, where bora is a casual way to say let's go and pro is a shortened form of para o, together forming an invite to head into a game. Online the shout was clipped into a very short soundboard button, which helped it spread in gaming chats, streams, and group messages as a quick call to play. Its stretched ending keeps it recognizable even without context, so it survives as a general hype reaction beyond its original gaming invite meaning.