the compound "squeaky-clean" is a vivid metaphorical extension from the mid-20th century. the first element, "squeaky," is an onomatopoeic adjective derived from the sound made by a clean, frictionless surface, such as glass or a wet plate, when rubbed. the second element, "clean," has ancient germanic roots, from proto-germanic klainiz, meaning clear or pure. the logic of the combination hinges on the auditory cue of a high-pitched squeak, which became culturally associated with the thorough cleanliness of an object just washed. this sensory metaphor was then applied figuratively to describe a person of impeccable character or a reputation utterly free from scandal, implying a level of purity so absolute it is audibly verifiable.

Structure: squeaky (sense) + clean (sense) squeaky (onomatopoeic origin, imitative of a high-pitched sound; functional role: modifier). clean (old english 'clǣne', meaning free from dirt or moral impurity; functional role: head adjective).