People understand it for themselves in relative terms.
I bumped into my deadbeat alcoholic former roommate a few days ago. He was stumbling down the sidewalk of a busier street, with his phone flashlight lighting up the way, hampered for several seconds by every curb, sipping on a pint of rail vodka as he went to the bar.
He announced "I'm not drunk" to me, because "drunk" to him is blacked out and slumped in a random location.
