There’s a commercial where the spokesperson says he’s not ‘lit’ (or is; I forget which). What does he mean? AFAIK, ‘lit’ means ‘drunk’.
Currently “lit” is a superlative, something that’s lit is very, very good. See also “fire.”
Urban Dictionary offers these definitions:
When something is turned up or popping…
John : did you go to that party last night
Daquan : yes that shit was lit
What millennials use when describing that is ‘fire’ or ‘dope’. Meaning cool or awesome.
Rapper #1: Ey, bruh want some weeeeeeeeeeeeeed?
Rapper #2: Yo, that shit lit, bruh lemme at it.
Over here, “lit up” would be a (now possibly outdated) metaphor for “drunk”.
It’s a general rule of linguistics that each generation must come up with its new words for something that’s good, so they can sneer at the oldtimers who use other words instead. It’s been happening for centuries.
Except for the coolest word ever, which of course is “cool”, and which seems to be used across the globe, among all social classes and groups, and for decades without losing popularity. /aside
Not sure I much to add on “lit”, except we used “lit up” in the mid ‘80’s, and this also had multiple meanings. If a boxer, or any person in a fight took a direct hit to the face, it would be described as him or her being “lit up”. Then there was also the drink/high reference. Also the band Lit named itself that in 1995. And the Buckcherry song “Lit Up” was from the late ‘90’s.
I’m pretty sure lit is the same as “off the chain”.
I recently received a text from a young man who wrote “bet” after I told him I was on the way.
I had to ask what that meant. It means cool.
Why on earth would someone say “Lit” when they could say “All that and a bag of chips?”
For me, it means either “totally awesome!” or “totally drunk!” depending on context. These days, I typically hear it more in the former, as in, “man, that shit’s lit!” (Just noticed that expression has already been given. I’m probably showing my age by still using the contracted “is” instead of just dropping it. 
I’m also still getting used to “bruh” vs “bro” or even “brah.”)
I believe it could also mean “totally high!”.
Ok, totally under the influence of something other than life.