It's a person's name or nickname ?A person may be called "Ace".
I asked this question because someone refers to one of the forumers as Ace (not in this forum). But his forum name is not Ace so I asked a question.
In which case it could be short for "ace hole" ("ace" sounding a lot like "ass")It can also be used ironically to mean someone who is showing off. It can also be used ironically to mean that someone is doing particularly poorly at something and being the opposite of an ace.
This is why we need context.