灯台下暗し
(Toudai moto kurashi; “Beneath a lamp is dark.”)

Definition:

It’s surprisingly easy to be ignorant about things or events close-by. We are most blind to what is under our very noses. “Go abroad to hear of home.” The image is based on a standing (rather than hanging) lamp, which naturally blocks its own light with its own base.

Breakdown:

This verbless, particle-free sentence begins with the noun 灯台 (toudai), literally “lamp pedestal.” The term can refer to a lighthouse, or to an old-fashioned interior lamp with a wick floating in a bowl of oil on a stand; in this case the latter is the image being invoked. This is followed by an unusual adverb: 下. In modern usage, the character 下 on its own is often pronounced shita and acts as a noun. But here it’s pronounced moto, and acts as an adverb meaning “under” or “below.” This adverb modifies the adjective 暗し (kurashi), “dark,” which appears in sentence-final form.

Notes:

Even knowing the meaning of the phrase, it would be easy to assume that the moto is homophone 元, “foundation.” This substitutes a noun for the adverb, though, and is considered an error. On the other hand, using 燈 (an alternate form of 灯) is acceptable.

My sources are pretty vague here, but this saying seems to trace back to the writings of Mencius, perhaps by way of a later text called the Helin Yulu, although the precise source is unclear.

Example sentence:

「マジかよ、家の妹が俺の親友とデートしてて、ずっと気づかない自分ってどんな馬鹿だよ」　「ドンマイ、ドンマイ。灯台下暗しってまさにそのことじゃないか」

(“Maji ka yo, uchi no imouto ga ore no shinyuu to deeto shitete, zutto kidzukanai jibun tte donna baka da yo!” “Donmai, donmai. Toudai moto kurashi tte masa ni sono koto ja nai ka.”)

[“Are you kidding me? My own sister and my best friend have been dating all this time; what kind of an idiot am I not to notice?” “Chill, man. It’s a textbook case of how it’s hardest to see what’s right in front of you.”]