Tamsin is an English diminutive of Thomasina, ultimately meaning twin.
Tamsin is a medieval English feminine diminutive of Thomas, and Thomas itself derives from the Aramaic ta'oma, meaning "twin." In the New Testament, the apostle Thomas — the one who demanded tactile proof of the Resurrection — gave the name its first great cultural currency in Christendom. By the Middle Ages, Thomas had become one of the most popular names in England, and feminine forms naturally followed: Thomasina, Thomasine, and in the west of England, particularly Cornwall, the affectionate contracted form Tamsin.
Cornwall is Tamsin's heartland. The name was so common in that region from the medieval period through the nineteenth century that it became essentially synonymous with a Cornish woman, appearing frequently in local records, ballads, and folklore. This strong regional identity gives Tamsin a grounded, almost earthen quality — it feels like a name that belongs to granite cliffs and fishing villages, to a Celtic fringe culture that maintained its own identity despite centuries of English pressure.
The Cornish connection also touches on Arthurian legend; Cornwall is the land of Tintagel and Tristan, and Tamsin carries a whisper of that mythic landscape. Through the twentieth century Tamsin enjoyed periodic revivals in Britain, never becoming fashionable enough to feel trendy but always retaining quiet admirers among those drawn to names with genuine historical depth. The British actress Tamsin Greig, known for her work in series like Black Books and Episodes, gave the name a warm contemporary face. Today Tamsin appeals to parents who want something undeniably English but not exhausted — a name that has been spoken for eight centuries without ever quite becoming common.