Billie is a tomboyish nickname name, part of the growing trend for using boyish nicknames for girls.
Billie is a possible retro choice for fans of jazz great Billie Holiday, born Eleanora. Other conceivable namesakes are The Wizard of Oz's Glinda--Billie Burke (born Mary William), silent screen star Billie Dove (born Bertha), actresses Billie Whitelaw and Billie Piper (born Lianne) and tennis legend Billie Jean King (born Billie). Another cultural reference is the Michael Jackson hit song Billie Jean.
Billie had double-digit popularity in the late 1920's and early 1930's.