The pre-drinking before heading out to a bar, or other event where overpriced liquor is the norm. "Pre-flight" enables said users to get liquored up before the event - so as not to spend as much money later on at the bar. Term used mainly by Marines, but other military types as well.
"Hey Smith, what's up for tonight?!" "Gate 2 man, but before we go, we're gonna pre flight here."
To prepare for a night of drinking at bars and clubs by drinking at home first, in order to already have a good buzz going before you get there. Cost effective.
Hey man, come by my place around 6 and we can preflight before we hit the club.