覆水盆に返らず
(Fukusui bon ni kaerazu; “You can’t put spilled water back in the saucer”)

Definition:

You can’t undo what’s been done. You can’t redo what’s been undone. You can’t uncrack an egg. As much as you may sometimes want to negate actions taken in the past or restore things that have been lost, doing so is often no more possible than retrieving water spilled onto the ground and putting it back in its former container. Due to its origin (see below), this phrase is especially appropriate when commenting on the impossibility of restoring human relationships that have ended.

Breakdown:

Despite being shorter than some noun-phrase sayings we’ve covered, today’s kotowaza is a full sentence, if one presented with extreme pithiness. We begin with the subject 覆水 (fukusui), “spilled water,” a noun apparently used nowhere else in Japanese. The particle that would mark it in proper modern Japanese is elided, and next we find the noun 盆 (bon), in Japanese a tray for serving meals but in this case a term referring to classical Chinese drinking saucers. The particle に (ni) marks the tray as the target of a motion, in this case the verb 返る, “to put something back.” In turn, the verb appears in imperfective form so that we can append the negative suffix ず (zu), which appears in sentence-final form.

Notes:

One of my sources asserts that replacing 返 with homophone 帰 is an error. And this makes sense, since the latter specifically denotes returning to one’s home. That said, you’re going to be seeing a lot of 帰 in the wild, if only because kaeru is no longer a common usage of the 返 character, allowing mis-writing or especially mis-typing to become an easy and common error.

I was surprised to learn that this saying also comes from classical China – from the Zhou dynasty, specifically. It’s said that in his youth, Lü Shang (also known to history as Jiang Ziya, among other names) didn’t work and instead sat and read books all day. This upset his wife, who left him. Later on he became powerful famous, working under the kings of Zhou and gaining a reputation as one of the greatest strategists in all of Chinese history. After his rise in society his ex came back and asked to become his wife again; in response he spilled a saucer of water on the ground and said that if she could put the water back in the saucer, he would accept her and they would re-marry. The story comes to us from a 1600-year-old text known as the Shi Yi Ji (拾遺記)。

Example sentence:

「どんなに謝っても後戻りはできないから、喧嘩をする時こそ言葉を慎もう。覆水盆に返らずよ」

(“Donna ni ayamatte mo atomodori wa dekinai kara, kenka wo suru toki koso kotoba wo tsutsushimou. Fukusui bon ni kaerazu yo.”)

[“You can’t take things back no matter how much you apologize, so you’ve got to especially watch your words during fights. You can’t put back spilled water, after all.”]