Reference in certain blues and rock songs. Exact meaning unknown. Now meant to refer to school playgrounds where social heirarchies and roles are exercised and determined. May also refer to urban schools where firearms and violence are common.
You better watch your six out on the killing floor, homes.
From old blues songs, particularly the Chicago style; a reference to the location in the slaughterhouse where the stock was killed and then butchered, hence a scene of danger, difficulty, bloodshed, etc.