What is the X in X-mas?
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].