Premise: Tabitha Anastasia "Tibby" Tomko-Rollins is a fictional character in the 2001 novel "The Sisterhood of the Traveling Pants" and the 2005 film based upon it. She is a member of the titular club, along with her friends Lena Kaligaris, Bridget Vreeland and Carmen Lowell. She was portrayed by Amber Tamblyn in the film.
Hypothesis: Tabitha goes by a nickname.