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.