"SCOTCH Fiddle: the Itch."
Dictionary of Cant and English Slang
by Nathan Bailey, 1736
The Scotch fiddle is both an obsolete slang term for a gonorrhea infection and for sex, and is also an obsolete obscene gesture.
Fiddle, in this case, is a reference to a violin,1 and to the in-and-out motion of the violin's bow; while this was in part a fun way to insult the Scottish, it was used primarily as a way to make clever references to sex in conversation, songs, and plays, as dancing, music, and intercourse make for good extended metaphors, and good opportunities for the foolish character to miss what the others are talking about. It is uncertain why this became specifically tied to gonorrhea, rather than remaining a more general sexual reference; in practice it was used to refer to either, but most frequently to gonorrhea.
Meanwhile, it was reported that Scotsmen in particular used to find the gesture of passing the right index finger in and out between the index and middle finger of the left hand an offensive gesture indicating that one has The Itch.2 As far as I am aware, this gesture is still likely to be interpreted as obscene, but no longer would be taken as a reference to a sexually transmitted disease.
1. Probably. There is also the oft-given explanation that during times of famine Scotsmen were obligated to live on a diet consisting primarily of oatmeal, resulting in itchy skin, and the habit of rubbing fingers between each other emerged from this; in this case, fiddle would have first emerged from the synonym for fidget, and been tied to The Itch in the confused idea that gonorrhea results in itchy fingers -- perhaps a confusion between gonorrhea and scabies. Language is chaotic enough that this could be true, and would help explain the gonorrhea connection.
2. as described in Proverbs, Proverbial Expressions, and Popular Rhymes of Scotland, collected by Andrew Cheviot, 1896. A short satirical essay in The Dialect of Craven, in the West-Riding of the County of York (Vol. 1) By William Carr, 1828, described it as using the fore-finger between the thumb and fingers of the other hand, but reported it as a reference to sex, rather than venereal disease.