実るほど頭の下がる稲穂かな
(Minoru hodo atama no sagaru inaho ka na;
“As modest as a ripened head of rice”)

Definition:

A description or proverbial example of great modesty befitting true greatness. Just as the ripened “ear” or “head” of rice on a rice plant will weigh down the stem so that it bends – reminding the Japanese of a polite bow – so will an exceptional person, a person of character, become increasingly modest even as their level of mastery or position in society rises. With great power comes great self-effacement, so to speak. There’s a passing similarity to “Still waters run deep,” although the nuance is different and the focus is on modesty.

Breakdown:

実る (minoru) is a verb, “to ripen,” “to bear fruit.” ほど (hodo) is a particle. Normally it indicates something’s extent, limits, or degree. Thus, we might expect these words to be translatable as “to the extent of fruiting/ripening.” But be careful; this kotowaza uses classical grammar. I believe that the meaning is closer to “As [A], increasingly [B].”

頭 (atama) is a noun, “head.” の (no) is a familiar particle – but here it is not the modern associative. Rather, it is a classical particle that filled the role now taken over by が (ga) as a subject marker. The verb that the subject performs is 下がる (sagaru), “to hang down.” 稲穂 (inaho) is a compound noun made with the characters 稲 (“rice plant”) and 穂 (“head of rice”), and – as you’d expect – means “head of rice” (or, by analogy with corn, an “ear of rice”?).

Finally, かな is a particle combination that in modern Japanese functions as a rhetorical question, a sort of “I wonder…?” But in classical usage it has an exclamatory function, equivalent to modern Japanese だなあ (da naa). Lafcadio Hearn, upon repeatedly encountering this particle combination while translating poems about butterflies for his collection Kwaidan, simply rendered it as an exclamation point.

Putting all this together, I’d say a good rendition is “As its fruit ripens, the rice plant bows its head” – or, in a more explicit and clunkier mode, “As it ripens, and to the extent that it ripens, the rice plant bows its head ever lower.”

Notes:

This is when I learned that the “multi-branched seed head” of a rice or similar plant is called a “panicle.” The term is derived from the Latin panus, meaning “ear of millet.”

下がる can be replaced with 垂れる (shidareru or tareru), meaning “to droop,” “to hang down,” “to weep,” and perhaps familiar from a previous kotowaza. A deeper restructuring that leaves behind the classical phrasing and tweaks the image slightly is 実る稲田は頭垂る (minoru inada wa atama taru), which has an entire rice field rather than a single plant.

The soft-spoken person of great ability is an ideal in many cultures, of course, but perhaps never so eminently enforceable as it was in feudal Japan, where angering the wrong person could end up with you being put to the sword.

Finally, it is possible that the classical turn of phrase has less to do with the age of this kotowaza than with the fact that it follows the 5-7-5 syllable pattern of a haiku. Even to this day, Japanese poetry will borrow the compactness and resonance of thousand-year-old grammar and syntax.

Example sentence:

「教師が学生の謙虚な態度を褒めるために手紙に書いて送った
『実るほど頭の下がる稲穂かな』」

(“Kyoushi ga gakusei no kenkyo na taido wo homeru tame ni tegami ni kaite okutta
“Minoru hodo atama no sagaru inaho ka na.’”)

[“Praising the student’s modesty, the teacher sent a note in which was written,
‘As it ripens
the humble rice plant
bows its head!‘”]