Definitions for undershortsˈʌn dərˌʃɔrts
Random House Webster's College Dictionary
short underpants for men and boys.
* (used with a pl. v.).
Origin of undershorts:
Underpants, type of underwear worn in skin contact with the hip portion of the body, small enough to be worn invisibly under shorts.
Origin: by analogy with underpants, etc.