The English word 'testis' is a direct borrowing from Latin, where it meant "witness." The anatomical usage is believed to originate from an ancient Roman legal custom. In solemn oaths, a man would place his hand upon his genitals, invoking them as a witness to his truthfulness, as they were considered the seat of male virility and the guarantors of lineage. This metaphorical shift from a legal witness to the male gonad was solidified in classical Latin medical texts. The word's journey into English is relatively recent, entering the language in the early 18th century as a formal anatomical term, preserving both the Latin form and its specialized meaning.

The word is a simplex loanword from Latin. It cannot be meaningfully segmented into smaller, independent morphemes within its current English spelling.