noun
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.
あの人はむっつり助兵衛だから、普段はおとなしいけど実はスケベだよ。
That person is a quiet lecher—he seems meek but is actually perverted.
彼はむっつり助兵衛で、誰もいないと思ってエロ本を読んでいた。
He's a lecher with a quiet exterior; he was reading a dirty magazine thinking no one was around.
むっつり is an adverb meaning 'sullenly' or 'quietly', often used to describe someone who is taciturn or keeps their feelings hidden.
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.