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."