How would explain "drift off" in the last passage? “I hate to bug you, but would you mind if I ran that mop across the floor there.” I started to get up to take the mop across the streak I’d left on the floor when I saw the janitor look down. “Why . . . there’s nothing on that floor, son. Don’t worry about it.” He reached out to pat my shoulder, but I turned back to the bar and grabbed my beer and finished it. I pointed to the bar and put more dollars on top of the ones the barman had not yet collected. “I’m sorry. I just thought . . . ,” and I must have drifted off because I did not see him move. I saw instead the mop-head swaying across the floor in narrow arcs where I’d been pointing. He walked off dragging the dirt gray of the mop’s fringes down the concourse behind him. How would explain "drift off" in the last passage? Does it mean he was drunk? PS: He was drinking the a bar. Thanks!