葦の髄から天井を覗く
(Yoshi no zui kara tenjou wo nozoku;
“To peer at the ceiling through a reed.”)

Definition:

Forming opinions and making decisions on a topic based on one’s narrow views and pre-formed beliefs rather than any sort of breadth or depth of experience. Making judgments about big issues based on little information, often in a self-serving way. Having a view as narrow as what can be seen through the hollow stem of a reed after the pith has dried up.

Breakdown:

The verb of this sentence is 覗く (nozoku) in sentence-final form. To 覗く is to look at something in or through a narrow space, such as through a keyhole or down in a ravine. It’s often translated as “peek” or “peep.” Next is the noun 天井 (tenjou), “ceiling,” marked by the particle を (wo) as the direct object of the verb. The location of the action is indicated by the particle から (kara), “from,” as occurring through the space of a 髄 (zui), “pith” or “marrow.” This in turn is joined by the associative particle の (no) to the noun 葦 (yoshi, although see below), the common reed.

Notes:

In some versions of this saying, the を may be elided, and/or 覗く may be replaced with the more generic 見る (miru), “to see.”

In other contexts, 葦 may be called ashi rather than yoshi. This is based on a pun: yoshi can also be 良し, “good,” and ashi can also be 悪し, “bad.” (See also 悪しからず, ashikarazu, “I’m sorry, but….”) Nobody wants to be adrift among the bads, so a practice arose of referring to reeds as yoshi.

This saying is the よ entry of the Edo iroha karuta set, and is mostly synonymous with the saying about the frog in the well.

Example sentence:

「葦の髄から天井を覗くほどの無知を遥かに極めた奴が大統領選に出馬できるなんて、信じられない。気持ち悪い」

(“Yoshi no zui kara tenjou wo nozoku hodo no muchi wo haruka ni kiwameta yatsu ga daitouryousen ni shutsuba dekiru nante, shinjirarenai. Kimochi warui.”)

[“For a guy, who is far and away more ignorant than someone peeking out at the world through a tiny keyhole, to be able to run for president – it’s hard to believe. It makes me sick.”]  😢