What is dumb genius?

A Mac Genius who doesn't know wtf he's talking about.

Like a guy that gives you the standard tech help line - "it's not our problem you have to go somewhere else to fix that." -

When you protest saying that he can, in fact, fix it, he asks two questions to a real Mac Genius and then fixs it in 2 minutes, which he could have done an hour ago, if he just knew what he was talking about.