A feminine variant of Thomas, possibly [according to Whyte] from the Gaelic form Tamhus, or altematively [from ONC] by contraction of Thomasina. Thomasina, from Aramaic "twin", was common before the Middle Ages, when the diminutive Tamsin became popular.  It is clearly related to the male pet name Tam, common in Scotland and other Gaelic communities.

Tamsin became confined to Cornwall for a long time but it has recently become more widely used.

In old documents in Latin, the form Thomasina, with appropriate case endings, was also used for Tamsin.