Fad

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mid 19th century (originally dialect): probably the second element of fidfad, contraction of fiddle-faddle. Compare with faddy.

wiktionary

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Of English dialectal origin. Further origin obscure. Possibly from Old English ġefæd(“order, decorum”) (compare Old English ġefæd(“orderly, tidy”), fadian, ġefadian(“to set in order, arrange”); or from French fadaise("a trifling thought"; see fadaise).

etymonline

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fad (n.)

1834, "hobby, pet project" (adjective faddy is from 1824), of uncertain origin. Perhaps shortened from fiddle-faddle. Or perhaps from French fadaise "trifle, nonsense," which is ultimately from Latin fatuus "stupid." From 1881 as "fashion, craze," or as Century Dictionary has it, "trivial fancy adopted and pursued for a time with irrational zeal."