To get drunk or start drinking before the hang over from last night has worn off.
Thus having something to "tie onto"
To tie one drinking session to another before the effects/consequences of the last have expired.
Guy 1: Hey, what's going on there?
Guy 2: I'm hungover as fuck, they had quarter pitcher night at Jake's last night.
Guy 1: So you're just going to hang out?
Guy 2: Fuck it lets tie one on.
Guy 1&2: *Crack open beers and commence drinking*