OED- "The ultimate origin is obscure." The supposition spelled out is that the germanic groups were trying to say the Roman word vidula, hence germanic fiedel and similar words. Sorcha, you should know that I comes before E except----- when it doesn't.
1377- In England, to fiddle was fythelen (or similar).
Orac, the violin IS a fiddle! There is the verb, to violin, which means to fiddle, "now only in familiar or contemptous use." (Taking liberties with the OED)