Functions as the past participle of the verb "rot", mainly archaic/literary.
pourriParticipe passé archaïque de « rot ». The grain had rotten in the damp silo before anyone noticed.
Le grain avait pourri dans le silo humide avant que quiconque ne s'en aperçoive.
Informal adverb meaning "badly" or serving as an intensifier (e.g. "You treated me rotten").
Colloquial intensifier placed before adjectives/adverbs.
They treated the new employee rotten.
Ils ont mal traité le nouvel employé.
It was rotten cold on the mountain last night.
Il a fait terriblement froid sur la montagne la nuit dernière.