Why do teenagers use the word "lit" to mean cool?
"Lit" appears to have changed its meaning gradually, from being illuminated, to be drunk, to being "happening", to being cool. Wiktionary gives these definitions:
Illuminated. He walked down the lit corridor. (slang) intoxicated or under the influence of drugs; stoned. (slang) Sexually aroused (usually of a female), especially visibly sexually aroused. (slang) Excellent, fantastic; captivating. We ordered pizza and we're going to stay up all night. It's going to be lit.