keep six
v. phr.— «“She begged Ray not to hang around with (name withheld by the Sun) anymore. Ray said he had one more job to do with him and all he had to do was to go out to this area, and that is the area where the body was found, and keep six, and that’s all (he had) to do.” Keeping six is a street term for keeping watch.» —“Inquiry bombshell” by Tamara King Winnipeg Sun (Manitoba, Canada) July 27, 2006. (source: Double-Tongued Dictionary)
Perhaps from the military “watch your six,” meaning “watch your back.” Six = back is determined by using the numbers on a clock face, where 12 is directly in front, three is to the right, six is to the back, and 9 is to the left.