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late 14c., fedele, earlier fithele, from Old English fiðele, which is related to Old Norse fiðla, Middle Dutch vedele, Dutch vedel, Old High German fidula, German Fiedel; all of uncertain origin.
Perhaps from Medieval Latin vitula "stringed instrument," which is perhaps related to Latin vitularia "celebrate joyfully," from Vitula, Roman goddess of joy and victory, who probably, like her name, originated among the Sabines [Klein, Barnhart]. Unless the Medieval Latin word is from the Germanic ones.
Fiddle has been relegated to colloquial usage by its more proper cousin, violin, a process encouraged by phraseology such as fiddlesticks, contemptuous nonsense word fiddlededee (1784), and fiddle-faddle. Fit as a fiddle is from 1610s.
late 14c., from fiddle (n.); the figurative sense of "to act nervously or idly" is from 1520s. Related: Fiddled; fiddling.