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1. The specific, individual people in a pair or group. I have a large number of people working for me, but I still try my best to remember who's who. Because of their uniforms and helmets, I can never tell who's who when I'm watching our kids play football.
2. A gathering of the most famous, important, or influential individuals in a larger group, field, or profession. The gala event is shaping up to be a who's who of the film industry. The expedition—the single most expensive ever commissioned—will feature a who's who of renowned scientists and researchers.
The outstanding or best-known individuals of a group, as in Tonight's concert features a veritable who's who of musicians. This expression comes from the name of a famous reference work, Who's Who, first published in 1849, which contains biographical sketches of famous individuals and is regularly updated. Its name in turn was based on who is who, that is, the identity of each of a number of persons, a phrase dating from the late 1300s. [Early 1900s]