The Collaborative International Dictionary of English v.0.48:

jockey shorts \jock"ey shorts`\ (j[o^]k"[=e] sh[^o]rts), n. pl.
   [from a Tradename.]
   A type of men's underpants without legs, fitting tightly and
   held by an elastic waistband; also called briefs.
   Originally a tradename, the term has become common for that
   type of underpants.
   [PJC]

WordNet (r) 3.0 (2006):

briefs
    n 1: short tight-fitting underpants (trade name Jockey shorts)
         [syn: briefs, Jockey shorts]