(Enjaku izukunzo koukoku no kokorozashi wo shiran ya;
“How could the swallow or sparrow understand the goose or swan?”)
People of lowly character or small ability don’t, and can’t, understand the thought processes and ambitions of those with good character or great ability. The story goes that there was a man who worked on a farm but harbored great ambitions. When his employer made fun of him, he responded with this phrase… and later became the king of Chu. There does seem to be a certain amount of classism and arrogance built into this phrase.
We begin with the compound noun 燕雀 (enjaku), literally “swallows and sparrows,” but more generally indicating “small birds.” What follows is a set phrase interjection: 安くんぞ (izukunzo), which essentially means “how~?” or “why~?” and sets the stage for the final や. Apparently the structure is derived from question word 何処 (idzuko, ancestor of modern どこ, “where~?”) plus directional or locational particle に (ni) plus linking particle ぞ (zo).
Next comes 鴻鵠 (koukoku), literally “wild geese and swans,” but more generally indicating “large birds.” The associative particle の (no) marks the noun 志 (kokorozashi), “will(power),” “intent,” “goal,” as the possession of these great birds. The particle を (wo) marks kokorozashi as the direct object of the verb 知る (shiru), “to know,” which appears in imperfective form and takes the negative suffix ず (zu), which is forced into prenominal form by the following bound particle や (ya), which indicates a sense of irony or makes the phrase into a rhetorical question.
Izukunzo may also be written in other contexts as 焉んぞ, but this kanji is no longer in common usage and does not seem to appear in modern forms of the saying. Alternate “spellings” include 烏んぞ (using the character for “bird”), 悪んぞ (“bad”), 奚んぞ (“servant”), and 寧んぞ (“preferably”).
This kotowaza may also be expressed in yojijukugo form as 燕雀鴻鵠 (enjaku koukoku).
As you perhaps guessed from the archaic grammar and kanji and the variety of ways to phrase the thought, this comes to us from classical Chinese literature – specifically, from our friend the Records of the Grand Historian (Japanese 『史記』 = Shiki).
(“Enjaku izukunzo koukoku no kokorozashi wo shiran ya to wa ie, kokumin ni jibun no kangae ya okonai wo kakushite bakari iru daitouryou wa shin’you dekinai shi, sassa to yamete hoshii.”)
[“Small birds may not understand the will of the great, but I can’t trust a president who’s always hiding his thoughts and actions from the public; I’d like him to resign ASAP.”]