Dictionaries are descriptive, not prescriptive. Just because it's in one doesn't mean it's proper English. I'm sure most dictionaries contain the word "ain't", but if a student uses the word, I ain't gonna give him credit for it. B^)
I think it's because kids don't really know what ain't is a contraction of (nice use, BTW).
I'm not really sure words like ginormous belong in the dictionary, smackdown seems fine because it's another form of beatdown. Ginormous and Crunk (especially crunk) are just nonsense words that somehow became popular, though it's nice that Conan O'Brien's made up swear word became so popular.
1: the act of knocking down or bringing down an opponent 2: a contest in entertainment wrestling 3: a decisive defeat 4: a confrontation between rivals or competitors
P.S. - I should've changed my icon sooner (see 1st post)...it's weird, and please don't take this as trivializing what has happened, but noticing little things in your life that jump out at you...like how I should have changed my Benoit DEAN pic avatar, or realizing I'll never be able to listen to OLP - "Whatever" the same way and deleting it off my iPod... Sorry to go off course.
Watch out, Regal, Finlay's gonna KILL YA Benoit now had two reasonably good chances of winning the World Heavyweight Title from Randy Savage, both of which ended because the Horsemen came in and caused a disqualification.