köken: From north-eastern Middle High German rase, probably from Middle Low German wrāse (“sod, turf”), though the two could also be cognate forms. Further origin presumably a variant of Middle Low German wāse, from Old Saxon waso, from Proto-West Germanic *wasō (“moisture, slime, muck”) which has been inherited as German Wasen. The intermitting r might be due to influence from dialectal forms of Brodem…
köken: From Old French bogre (“heretic”) (1172), also bogresse (“person who indulges in unnatural debauchery”) (1260), from Late Latin Bulgarus (“Bulgarian”), from Old Church Slavonic блъгаринъ (blŭgarinŭ). Doublet of bulgare and boug.
köken: Borrowed from French gazon, from Middle French gazon, from Old French gason, wason (“piece of ground covered with grass, turf”), from Frankish *waso (“ground, turf”), from Proto-Germanic *wasô, *waisǭ (“moisture; ground”).
köken: From vireō (“to be green, verdant”) + -tum (suffix forming nouns for stands of vegetation). The -c- would be inserted by analogy to forms like frutectum (“thicket, bush”), cārectum (“reed bed”) that are derived from noun stems ending in /k/.