'testicular' originates from Latin, specifically the word 'testiculus', a diminutive of 'testis', where 'testis' originally meant 'witness' and 'testiculus' meant 'little testis'.

'testiculus' passed into Old French as 'testicule' and into Middle English as 'testicle'; the adjective 'testicular' was later formed in English from 'testicle' + the adjectival suffix '-ar'.

Initially the root 'testis' meant 'witness'; over time the word came to denote the male reproductive organ ('testicle'), and 'testicular' evolved to mean 'pertaining to the testicles'.