From Latin 'testis' (witness) — sharing a root with testament, test, protest, contest, and (awkwardly) testicle.
To give evidence as a witness in court; to declare solemnly.
From Latin 'testificari' (to bear witness), from 'testis' (witness). The connection between 'testis' (witness) and 'testis' (testicle) has fueled a persistent legend that Roman men swore oaths by grasping their own testicles. Key roots: testis (Latin: "witness (third-party observer)").