When consuming alcoholic beverages, Stack-a-Lackin' is the point in time (that usually doesn't last long) where you are past having a buzz, but not yet to the state of being drunk. It is usually hard to tell when exactly you are Stack-a-Lackin', but your best bet is just to say you are when everyone else starts saying it.
Also known as having a 'heavy buzz'.
Jethro: "Hey man, you d-dr-drunk yet?"
Stumps: "Nah man, I'm Stack-a-Lackin'!"
Edgar: "Duuuude, I'm Stack-a-LACKIN'!"
Allen: "Shiiiit, me too, brother!"
Poe: "Oh snap!"
Monty: "This party is Crack-a-lackin
G-Dawg: "Jeah nugga, and yours truly be Stack-a-lackin!"