This is very far from the truth! The Romans certainly did not swear oaths on their private parts.
The confusion can be cleared up by looking at the etymology of the word ‘testify.’ It originates from the Latin word ‘testis’ meaning ‘witness.’
Note there is a difference between testis and testes. Later the word developed into ‘testificari’, still meaning ‘a witness’. Only in late Middle English was the word ‘testify’ used, which was derived from the root Latin word ‘testificari’.
Now, you might wonder where the word ‘testicle’ originates from. ‘Testiculus’ is the diminutive of the Latin root word ‘testis’ and means ‘witness of virility.’ The plural is testes. Clearly both these refer to a witnessing of some kind, but testifying certainly did not come from men placing their hands on the testicles of others.
To clear up any further confusion (if there could still be any) ‘virility’ originates from the Latin word ‘virilitatem’ (from virilis), which means power of procreation. Therefore ‘testicles’ directly translated would mean ‘witness of (a man’s) power of procreation.’
