The Collaborative International Dictionary of English v.0.48:

Testicle \Tes"ti*cle\, n. [L. testiculus, dim. of testis a
   testicle, probably the same word as testis a witness, as
   being a witness to manhood. Cf. Test a witness.] (Anat.)
   One of the essential male genital glands which secrete the
   semen.
   [1913 Webster]

WordNet (r) 3.0 (2006):

testicle
    n 1: one of the two male reproductive glands that produce
         spermatozoa and secrete androgens; "she kicked him in the
         balls and got away" [syn: testis, testicle, orchis,
         ball, ballock, bollock, nut, egg]