A term used by inner-city youth, usually in a gang and oftentimes African-American. Describes a quick, spontaneous shooting (hence 'flash') of an individual (or perhaps of a small group) by a gang of African-Americans (hence 'mob.')
The term 'flash mob' originated as some kind of strange gimmick where net-savvy nerds met in a public place to do something ridiculous, then quickly disperse.
However, 'flash mob' has gone from being something to laugh at to something to fear. Five or six blacks (or occasionally Mexicans, Asians or even white trash) with guns will jump a rival gang member, someone they have beef with, or even a random person, usually in a dark alley or parking lot, shout "flash mob!" and just open fire. A flash mob could also be a drive-by shooting where there are several shooters.
"Flash mob, nigga!" (blast BLAST boom boom POW)