early 15c., alteration of testicule (late 14c.), from Latin testiculus, diminutive of testis "testicle" (see testis). Old English had beallucas (see ballocks) and herþan, probably originally "leather bag" (compare heorþa "deer-skin"). The commonest slang terms for them in other languages are words that mean "balls," "stones," "nuts," "eggs."
word-formation element meaning "pertaining to, of the nature of," from Latin -arem, -aris "of the kind of, belonging to," a secondary form (by dissimilation) of -alis, used after syllables with an -l- (such as insularis for *insulalis, stellaris for *stellalis).
<a href="https://www.etymonline.com/word/testicular">Etymology of testicular by etymonline</a>
Harper, D. (n.d.). Etymology of testicular. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/testicular