A method of holding a firearm, e.g. handgun, submachine gun etc., in such a manner that the weapon is mostly or fully horizontal. So named for the gangsters who supposedly demonstrate this technique in gunfights. Frequently combined with dual wielding.
Shooting a gun in this way has no practical benefit under most circumstances and makes proper aiming very difficult. However, the style has become somewhat popular in rap culture and among street criminals (who do not often use the gun sight) due to its portrayal in U.S. film and television since the 1990s, which in turn was inspired by Hong Kong action cinema beginning in the '70s and '80s. It is believed that Hong Kong actors needed to hold their guns in this way because some Chinese-made automatics couldn't eject shell casings reliably.
It is also known that this type of grip was sometimes used during the Chinese Civil War of the '30s and '40s with full-auto versions of the top-ejecting Mauser C96 "Broomhandle" pistol, which had an extremely high rate of fire and significant muzzle climb when fired normally. When held in what's now known as gangsta style and fired, the muzzle would instead "climb" sideways and thus sweep across everything in front of the shooter. A similar technique was sometimes used with submachine guns during World War II, but as they were significantly bulkier and heavier than the C96 it was considerably more awkward.
Side grip at Wikipedia