(Note: this definition, though inadvertently, was taken from Coca-Cola's choice of branding for its regular soda, Coca-Cola Classic, and it's Diet Coke. See example.)
synonyms: nasty, terrible, awful, lame.
antonym: classic, as such: "classic is good, diet is bad"; fly; sweet
During the unloading of a delivery truck, most of a stack of Coca-Cola soda cans crashed to the floor in the back of a grocery store after being hit by an arriving, full palette of other goods. One store worker, trying to save surviving cases of cans from the carnage, took a lot of soda spray to his clothes, eyeglasses and skin.
A coworker, just arriving on the scene and comprehending the accident, joked, "Wow, that was classic."
"Nope," replied the worker, looking at his wet, sticky work clothes. "It was diet. It was definitely not classic."
Realizing the double entendre, a slang term was born (at least in those parts).
Steve: Hey want to go eat some food?
Janet: No thanks, I'm on a diet.
I am sooo like fat!!! Ima go ona diet for realz for realz. No more coco for me, ya heard!
uMMMM...them chips look good as hellz.... fuck it, im already fat, why dont I live a little?? (chomps on chips)