Thomasina, Thomasine, or Thomasena is the feminine form of the given name Thomas, which means "twin".
Thomasina is often shortened to Tamsin.
Tamsin can be used as a name in itself; variants of Tamsin include Tamsyn, Tamzin, Tamsen, Tammi, Tamzie and Tamasin.
The version "Tamsin" is especially popular in Cornwall and Wales.
Along with Tamara it is the ancestor of "Tammy".