I had occasion the other day to look up the origin of English tress and found that there are various theories. What’s known is that English took the word from Old French tresse. That noun meant ‘a braid of hair,’ a sense that English tress still has, although the American Heritage Dictionary marks it archaic. The modern sense of tress, as given in the Oxford Dictionaries, is ‘a long lock of a woman’s hair.’
But back to the origin of the word. The American Heritage Dictionary says that Old French might have inherited tresse from Vulgar Latin *trichia, tricia ‘a rope, braid,’ which the Romans had taken from Greek trikhiā ‘a rope,’ based on thrix or trikh- ‘hair.’ The Dictionnaire historique de la langue française offers that hypothesis and a couple of others. One, proposed by Gamillscheg, would go back to Frankish *thréhja, from the same root as Germanic words having to do with turning (and possibly related to Latin torquere). Another theory, proposed tentatively by Corominas, sees the Old French word as a relative of Spanish trenzar, which would have arisen from the Latin verb tertiāre, based on tertius ‘third.’ The semantic connection, of course, is that hair is typically braided into three strands, each of which makes up a third of a braid. As for Spanish trenzar, the DRAE follows the same semantic line and reconstructs a phonetically dictated Vulgar Latin *trinitiāre, which would have been based on Latin trīni ‘three each.’
In summary, it’s unclear whether Spanish trenza and English tress are cognates or merely one more pair of etymologically unrelated words, like día and day, that coincidentally (but strikingly) sound similar and mean the same thing.
© 2015 Steven Schwartzman