Gang Signs

Within our contemporary society, there is also what I have often called a secret society. It has emerged from the world of gangs. These people, of various ages and intentions, have formed a system of communication that supposedly will keep most of us "in the dark" about their activities and ways of life. If you have a child, friend or other young person who is becoming, or has become involved with such activities, you would do well to make yourself aware of what is involved.

http://zimmer.csufresno.edu/~haralds/htmlfiles/gang-signs.html

This one has several signs - apparently posted by various people & police depts.
http://www.knowgangs.com/gang_resources/handsigns/menu_001.htm

We especially noted the bottom right one on this site.
http://www.enhancementcourses.edu/gangs/gang_signs.htm