A brainiac is a very intelligent person.

// Her ability to solve almost any puzzle within minutes secured her place as the brainiac of the family.

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"In this modern, adult-oriented take on the classic 'Scooby-Doo' franchise, the series follows the origin story of Velma Dinkley (Kaling), the brainiac of the Mystery Inc. gang. After a corpse is found in her high school, Velma teams up with Daphne (Constance Wu), Shaggy (Sam Richardson) and Fred (Glenn Howerton) to solve the murder." — Michaela Zee, Variety, 21 Dec. 2022

As Superman fans know, Brainiac was the superintelligent villain in the Action Comics series and its spin-offs. His name is a portmanteau of brain and maniac. You don't need x-ray vision to see the connection here—etymologists think Superman's brainy adversary is the likely inspiration for the common noun brainiac. The term was not coined right away though. The comic-book series was launched in 1938 and the character Brainiac debuted in 1958, but current evidence doesn't show general use of brainiac to refer to a superintelligent person until the 1970s.