Used to form adjectives from nouns, reduplications or imperfective form of verbs. An adjective derived by this suffix implies a thorough and certain possession of the quality of that noun.
From nouns;おとな+しい=おとなしい It mean "quiet", "gentle".
From verbs;いそが+しい=いそがしい It mean "busy".
From Japanese original reduplications;2×みず+しい=みずみずしい It mean "fresh","juicy".
From reduplication of Chinese origins;2×騒(そう)+しい=騒々しい(そうぞうしい) It mean "noisy".