This is one of my more favorite and more commonly quoted kotowaza. I’m not sure how it took me so long to get around to it. Time flies, I guess. 8^D

光陰矢の如し
(Kouin ya no gotoshi; “Light and shadow are like an arrow”)

Definition:

“Time flies like an arrow.”

Breakdown:

光 is light, in this case sunlight. 陰 is shadow, but also the yin of yin-yang (陰陽) and therefore representative of the moon. In other words, while literally 光陰 (kouin) means “light and shadow,” it’s probably closer in meaning to 月日 (tsukihi, “months and days”), another term denoting an extended span of time. (It can also be pronounced gappi, mean “month and day,” and simply refer to the date.)

Somewhat unusually, the compound 光陰 is immediately followed by another noun without anything else in between: next comes 矢 (ya, “arrow”). After that, the associative particle の, and finally the auxiliary verb 如し (gotoshi, “just like,” “as”).

You could even say that “Light/shadow, arrow’s likeness” is an accurate semantic representation, although in English grammar this won’t fly, ha ha.

Notes:

The idea of time being represented by “light and darkness” has an impact on me, and I immediately think of Genesis: “and the evening and the morning were the Nth day.” It makes sense, of course; the rhythms of sunlight and moonlight were the first things that gave us a measure of time, and proved so useful that days, months, and years remain with us even now. And on that note, I have to keep in mind that 光陰 is most likely “sun and moon” rather than “light and darkness.” Ah well.

My sources note that using the phrase to refer to the speed of some human endeavor is incorrect: this kotowaza points to the inevitable march – sprint, really – of time itself.

Example sentence:

「あああ、ヤバイ！あっという間に夏休みが終わった」　「そうだね。光陰矢の如しだなぁ」

(“Aaa, yabai! A tto iu ma ni natsuyasumi ga owatta.” “Sou da ne. Kouin ya no gotoshi da naa.”)

[“Aagh, this is bad! Suddenly summer vacation is over!” “Yeah. Time sure flies.”]