Both literally and metaphorically… not that a fundoshi actually covers much of the butt….
(Giri to fundoshi kakasarenu;
“Underwear and courtesy cannot be left lacking”)
In the old days, it was acceptable for men doing hard labor to strip down (and for small children to go essentially naked, with perhaps a smock on their fronts), but the minimum acceptable clothing for adult males was a fundoshi, a loincloth made by wrapping, tying, and/or tucking a long strip of cloth. This saying asserts that properly observing one’s obligations and the strictures of decent behavior is just as necessary as that standard of physical “decency.”
We begin at the end with the verb 欠く (kaku), “to lack.” It appears in imperfective form as 欠か (kaka) and takes the causative suffix す (su), itself in imperfective form as さ (sa) and taking the passive suffix る (ru), which is also in imperfective from as れ (re) and takes the negative suffix ず (zu), which is in – did you guess it? – conjunctive form as ぬ (nu). The resulting pileup is 欠かされぬ, “(can)not be made to be lacking” – or in simpler terms, “indispensable.”
Any particles that might point to the verb’s function are elided, but this verb structure is a predicate and the preceding phrase uses the particle と (to) to show that the verb applies to each of its nouns: 褌 (fundoshi), “loincloth,” and 義理 (giri), a complicated term that can refer to duty, honor, social obligations, or common courtesy.
義理 puts a lot of weight on the sense of owing someone something (such as a form of behavior), often more for the sake of social cohesion as a whole rather than for them personally. The phrase 義理を欠く (giri wo kaku) on its own refers to failing one’s social obligations. See also 義理の～ (giri no ~) appended to familial terms for one’s in-laws, or the practice of 義理チョコ (giri choko), chocolate given to work associates and superiors on Valentine’s day for the sake of soothing their feelings rather than any romantic interest.
This is one of two possible き (ki, in this case voiced as gi) entries in the Kyoto iroha karuta set. The Osaka set uses the contracted form 義理と褌. Other variations replace the verb phrase with せねばならぬ (seneba naranu), “shouldn’t not do,” or 裸になってもせずには居られぬ (hadaka ni natte mo sezu ni wa irarenu), “can’t exist without doing, even if (otherwise) naked.” And of course the base form will allow a comma between the noun phrase and a verb phrase, if you feel like using one.
The ultimate source of this phrase is unclear, although it is apparently attested in Edo-era reference works such as the 『譬喩尽』 (Tatoe-dzukushi, literally “an exhaustive list of figurative speech”) and the 『諺苑』 (Gen’en, literally “garden of sayings”).
(“Saikin, rouka de no aisatsu de sae mendou kusaku natte kita kedo, giri to fundoshi kakasarenu to iu shi, yappari meeru no henji nado wa fundoshi wo shimete kakaranai to ne.”)
[“Recently even just saying ‘Hi’ to someone in the hallways has started to feel like a pain in the butt. But courtesy is the bare minimum requirement for decency, as they say, so I’d better gird my loins and answer some emails.”]