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1834, "hobby, pet project;" 1881 as "fashion, craze," perhaps shortened from fiddle-faddle. Or perhaps from French fadaise "trifle, nonsense," ultimately from Latin fatuus "stupid."
flavin adenine dinucleotide
["FAD, A Simple and Powerful Database Language", F. Bancilon et al, Proc 13th Intl Conf on VLDB, Brighton, England, Sep 1987].