köken: 18th century, from English frock. Ultimately from Proto-Germanic *hrukkaz and thus a doublet of Rock. The very open English [ɒ] was substituted with German [ɑ] (as it was then predominantly spoken).
köken: Derived from Old French froc, from Frankish *hrokk, from Proto-Germanic *hrukkaz, from Proto-Indo-European *rukn-, *ruk-, *rug-, *ruǵ-. Doublet of rok.