Ok, the 'spanish coin' came from Sicily. Here is the etymology:
Ducat: name of a silver coin issued in 1140 by Roger II of Sicily, as Duke of Apulia, bearing the inscription R DX AP, i.e. Rogerus Dux Apuliæ; according to Falcone de Benevento ‘monetam suam introduxit, unam vero, cui Ducatus nomen imposuit’
ducat is shortened from ducatus
how is this motorcycle related? Ducatus, Ducati... yes, you see it now, don't you? Related words. And, oddly enough, it will cost you some ducats to maintain your ducati.
nuts, eh? (see nuts, above)