rubbish n.

"rough, irregular stones broken from larger masses," especially "waste fragments from the demolition of a building, etc.," late 14c., robeyl, from Anglo-French *robel "bits of broken stone," which is of obscure origin, apparently related to rubbish "waste fragments" [OED], but also possibly from Old French robe (see rob). Middle English Compendium compares Anglo-Latin rubisum, robusium.

adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.

The -ish in verbs (abolish, establish, finish, punish, etc.) is a mere terminal relic from the Old French present participle.