From the experts:
Tamsin is an offbeat name occasionally heard in Britain and just waiting to be discovered here. U.K. actress Tamsin Greig is a star of the show Episodes, Tamsin Olivier is the daughter of Joan Plowright and Sir Laurence Olivier..
Tamsyn is the alternative medieval Cornish spelling. Other spellings are represented by actresses Tamzin Outhwaite and Tamzin Merchant, and by writer Tamasin Day-Lewis, sister of Daniel.
Tamsin is a 1999 fantasy novel by Peter Beagle.