quiet lecher; lecher with a reserved exterior
Describes a person who is secretly lecherous but outwardly quiet, reserved, or seemingly uninterested in sexual matters. Often used in a colloquial, slightly old-fashioned or humorous tone.
That person is a quiet lecher—he seems meek but is actually perverted.
He's a lecher with a quiet exterior; he was reading a dirty magazine thinking no one was around.
Compound of むっつり (sullenly, quietly) and 助兵衛 (lecher, from すけべえ, itself a sound shift from 好き者). The exact historical derivation is uncertain, but the term paints a picture of a person who is secretly lecherous despite a quiet demeanor.