A phrase taken from the jargon of fighter pilot
s used to describe looking behind you. In air to air combat
, pilots use the number positions on a clock face
to describe the position of objects around them, including enemy
planes. Twelve o'clock would be dead ahead, and six o'clock directly behind you. Used skillfully, it is a very quick way to describe positions.
"Check six" is also a slang term that means "cover your ass".