The Collaborative International Dictionary of English v.0.48:

jockstrap \jock"strap`\ (j[o^]k"str[a^]p), n.
   1. A supporting undergarment for the external genitals worn
      by men engaging in vigorous athletic sports or strenuous
      exercise; called also athletic supporter and jock.

   Syn: jock; athletic supporter.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

athletic supporter
    n 1: a support for the genitals worn by men engaging in
         strenuous exercise [syn: athletic supporter, supporter,
         suspensor, jockstrap, jock]