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.