A person that can have many drinks, become drunk, and still function perfectly normal. It's hard to tell whether or not this person is drunk unless you ask them to recite something.
Girl: I was so drunk last night but I had a normaltexting conversation with my boyfriend.
Other girl: That's weird but you might be a functional drunk.
"Man good thing we got out of that party. Shit was going whack!"
"I know dude. Fuck that sprawl. I'm glad we're going home to play some super smash bros. for the Wii U."