(Se ni hara wa kaerarenu; “The belly can’t take the place of the back”)
If something is vitally important, then other things may need to be sacrificed on its behalf, even if they are also important in their own right. Sometimes you’re forced to choose the lesser of two evils or use a gambit. In a swordfight you don’t want your back to get slashed or stabbed, but it would be even worse to turn and protect your back by taking the hit in your vitals. In a darker sense, this phrase may be used to justify “looking out for number one” – focusing on protecting and taking care of yourself without spending any time considering how others are faring.
The ruling verb in this brief phrase is 代える (kaeru), “substitute,” “replace,” in a negative potential form. The particle に (ni) identifies the thing (not) being substituted for as the noun 背 (se), “back” (as in the posterior half of the human body). And the thing (not) being substituted in is the noun 腹 (hara), “belly”; this is marked by the particle は (wa), apparently simply for emphasis. (An unemphasized version would probably use を, wo.)
Back up a bit. The negative suffix in use here is ず (zu); ぬ is its prenominal form. Yet its use here occurs at the end of a sentence rather than before a noun; there isn’t even an implied noun. What’s going on?
It seems that it’s nothing more than codified confusion. There’s another ぬ out there, a perfective suffix that might be translated as “~ has ~ed.” (For example, this is what we see in the Ghibli movie title 『風立ちぬ』).
The whole phrase may be contracted to just 背より腹 (se yori hara, “belly before back”). Some versions may expand 背 to 背中 (senaka), use kana か or kanji 替 (both ka) in place of kanji 代, and/or replace the final ぬ with modern negative ending ない (nai), all without any change in meaning. However, replacing the particles to make 背を腹に (se wo hara ni) reverses the image and is an error.
This is the se entry in the Edo iroha karuta set.
Trivia of the day: the Hebrew phrase בטן גב (beten-gav), literally “back-belly,” refers to lounging around, especially sunbathing.
(“Baito mo suimin mo gisei ni dekinai shi, se ni hara wa kaerarenai no de, kongakki wa zannen nagara Supeingo no jugyou wa otosasete itadakimasu.”)
[“I can’t sacrifice work or sleep, and something must give, so I’m requesting permission to drop my Spanish class this semester.”]