In the US military, a person with red hair. This comes from comparing them to a "tracer", a firearms
cartridge that has a bit of burning magnesium at the base of
the bullet, which burns and can be seen as the bullet flies through the air.
The term "tracer" can refer to either their hair resembling the red path of a tracer bullet flying, or the fact that tracer ammunition in
NATO has the tips of the bullets painted red so that specialized ammo can be visually identified.
(to a redheaded coworker) Hey, you over there by the water-bull,
getcher ass over here! No, not you Sanchez, that tracer
ass-hat next to you... that redheaded
assclown.