look up any word, like swoll:
 
2.
to watch over; be a lookout for
"stay by the window and keep six for any cops."
by bmoc November 06, 2003
 
1.
To hang back and watch out for somebody when they are going into a potentially dangerous situation. As in before a large illegal transaction, or while conducting a robbery.

Has nothing to do with police being 5-0. Actually derived from military communications. When an infantry squad is moving, one member may be assigned to "watch the squad's 6", meaning their 6 o'clock (behind them).
Alright guys, there's supposed to be no guns inside the pool hall, so you stay out here and keep six for me. If I you hear shots or I'm not out in 20 minutes, come in blasting.
by Wasteland Soldier January 22, 2007
 
3.
to be on the lookout
For example a robbery is being committed and one robber tells the other to go outside and KEEP SIX
by SonnyStunner February 05, 2012
 
4.
To keep a look out for police, derived from cops being 5.0, so six means you're one up on the cops.
I'll park here and roll this up if you keep six!
by vijay December 03, 2003