Functions as the past participle of the verb "rot", mainly archaic/literary.
Definitionen
Archaic past participle of "rot"; having undergone rot or decay. The grain had rotten in the damp silo before anyone noticed.
Informal adverb meaning "badly" or serving as an intensifier (e.g. "You treated me rotten").
Definitionen
In a very bad, unpleasant, or unsatisfactory manner; badly. They treated the new employee rotten.
Colloquial intensifier placed before adjectives/adverbs.
Definitionen
Very; extremely (used colloquially as an intensifier before adjectives or other adverbs). It was rotten cold on the mountain last night.