Rex is the Latin word for "king", once a royal title rather than a given name; it caught on in the English-speaking world in the 19th century and sounds brisk and modern despite its ancient origin. Actor Rex Harrison and the Tyrannosaurus rex are the first things most Americans associate with it. Rex returned to the US top 1000 in 2003, peaked at number 607 in 2017 and ranks number 859 among boys in 2025.