Clock

verb

clock

1. To stay aware of what happens around you. "You're the one that I'm clocking" -- Ol' Dirty Bastard (Raw hide [1994])
2. To earn. "I clock ducats" -- Public Enemy (??? [??]).
3. To always watch, always have or to always be into something. "clockin' a grip" -- Dr. Dre and Snoop Doggy Dogg (Nuthin' but a "G" Thang [1992]).
4. To hit or knock someone out, as in "he clocked him".

Probably comes from "he got his clock cleaned".