A fanatical fan, especially one who is extremely devoted to a sports team, celebrity, or fan community.
Likely a blend or shortening of 'fanatic' + 'dad' or similar suffix, or possibly influenced by 'fandad.' This appears to be informal/slang with uncertain origin, emerging from fan culture.
Words like 'fantad' show how fan communities invent their own vocabulary—what starts as insider slang among super-fans eventually gets documented, and linguists have to track how internet culture creates new words faster than traditional dictionaries can keep up.
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