(Imo no nieta mo gozonji nai;
“Doesn’t even know whether a potato is boiled”)
This phrase describes someone from such an overprotective, sheltered existence that they can’t even tell whether a potato is raw or boiled. Potatoes are associated in the Japanese mind with boorish country “bumpkin” living – but even urban sophisticates or sophisticated urbanites are supposed to be able to distinguish this basic a difference.
This saying is thematically similar to one previously introduced here, about a frog in a well, but more direct and mocking in tone.
Alternately, the saying can be used to refer to someone not paying attention, as a chef who doesn’t notice that the potatoes have boiled already and can be taken off the fire.
芋 (imo) is a potato, or rather any of a variety of edible tubers and corms including potatos, yams, and taro. The verb 煮える (nieru) is the intransitive counterpoint to 煮る (niru), “to cook (by boiling).” Here it is in past tense. The particle の (no) in between the initial noun and verb is not in its familiar associative function, though: this の is acting like the subject-marker particle が (ga), a vestigial function that was more common in older Japanese grammar. And the noun-verb group is marked by も (mo) as an intensifier. Modern Japanese would probably put the interrogative particle か (ka) before the も to set off the subordinate clause.
Next comes… well, technically the verb of the sentence, but in a form that probably seems counterintuitive to most learners of Japanese. As noted below, the unmarked verb here would be 知る (shiru), “to become aware of” or “to know.” Despite the mocking content (or perhaps to highlight the mocking content) of the phrase, though, the verb is usually replaced by the honorific equivalent 御存じ (gozonji). To this we add the negative suffix ない (nai). Note that ない acts like an adjective, meaning that the place of the verb has in essence been taken by a noun-adjective pair.
The honorific prefix 御 can also be written as hiragana ご. The whole honorific verb can be replaced with the unmarked version, 知らない (shiranai).
This kotowaza is included in the Edo iroha karuta set as the ゐ entry.
(“Kore, kakunin shitain desu ga, paatonaa wa onore ga imo no nieta mo shiranai tte iu no de komarimasu.”)
[“I want to check this, but my partner says they don’t know their ass from a hole in the ground. I’m at a loss.”]