Uncredible is the opposite of Incredible, meaning that something is so completely abysmal as to defy any sort of description. Also can be used in sarcastic remarks, so as to say something that slips past most people's notice, but listeners might catch. A companion word to Underwhelming.
Rachel: So I was all like, this guy is TOTALLY hot, I think that he is AMAZING! He has all of Dylan's albums, on VINYL! Oh, Emm, Gee!