C'ain't is an abbreviation of two abbreviations usually used in more Southern Regions of the United States. It's a combination of Can't and the improper word Ain't. It's meaning is double sided. It's used in the sense of you can't do it and you won't do it.
What complete twats call themselves in the mistaken belief that they are somehow cool or intelligent.
"Lee Martin sits in his fetid bedroom all day wanking over his Hornby train set and making up bullshit stories whilst covered in dog fleas and nursing bruises from the last beating his fat pikey dad gave him, and he still has the nerve to call himself a whizzkid"