A jockstrap was a type of underwear.
In 2009, Gwen Cooper found a Chameleon pretending to be Jonathan Dee's corpse wearing a jockstrap in the middle of the woods. Gwen said that he could be wearing them due to sports, but Martha Jones asserted that "Jonathan" was probably meeting someone for sex. (AUDIO: Dissected)
In 2593 whilst searching Sunless uniforms Emile Mars-Smith wonders if jockstraps formed part of the uniforms. Although when Tameka Vito saw him naked she said he didn't need one. (PROSE: Beyond the Sun)