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'.

When you eat dirt for the first time and leave your family to build a shack in the woods

a state of comfortable intoxication from alcohol (usually beer) and possibly marijuana; the beautiful place between "tipsy" and "drunk" or "crossed" at which an individual feels his absolute best during his or her evening.

after another rip off the joint, one might say he or she is "stack-a fuckin-lackin"