In the 1940's the author Maud Hart Lovelace wrote a series of books about 2 girls, Betsy and Tacy, but I don't know if Tacy was a shortened form of a longer name.

If memory serves, Tacy was a not uncommon Welsh given name from the 1600s on....not a shortening of another name. S. -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.1.392 / Virus Database: 268.5.5/335 - Release Date: 5/9/2006