Billie is a feminine spelling of Billy, the pet form of William, and so carries its meaning of "resolute protector". Jazz singer Billie Holiday and tennis champion Billie Jean King made it an American classic, and pop star Billie Eilish has brought it back for a new generation. Billie ranked number 1305 in 2000 and number 2016 in 2015, then rose sharply, entered the top 1000 in 2021 and reached number 691 in 2025, its best rank in decades.