Thus having something to "tie onto"
To tie one drinking session to another before the effects/consequences of the last have expired.
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*
It is thought that its origin dates back to the wild west in the 1800's, where a cowboy would have to tie up his horse to a hitching post before he could go into the saloon and get drunk.
A British slang for this term, "tie a bun on", also means "to get drunk", however nobody is quite sure how someone came up with that one...