Creaky is from 'creak' (an imitative, onomatopoeic word for a harsh grating sound) + the adjectival suffix '-y'. The immediate etymological form is onomatopoeic creak. The suffix -y forms an adjective meaning characterized by the base, so the English form is shaped by recognizable word formation as well as by its inherited or borrowed base. In present use as an adjective, it means "making a harsh squeaking sound; stiff and decrepit", keeping the historical formation close to the word's ordinary semantic work. Related words include creak; derived forms include creakily and creakiness.