c. 1400, robous, "waste, broken, or worn-out material," especially "rubble from the demolition of a building, etc.," from Anglo-French rubouses (late 14c.), a word of unknown origin. There are said to be no apparent cognates in Old French; OED says "app. related in some way to rubble."
The spelling with -ish is from late 15c. As "any useless or worthless stuff" by c. 1600. The verb sense of "disparage, criticize harshly" is attested by 1953 in Australian and New Zealand slang. Related: Rubbished; rubbishing.