(Juubako no sumi wo tsutsuku;
“To poke at the corners of a juubako”)
To fuss over trivial details; to nitpick; to carp endlessly about fine points that don’t actually make a difference. The image is of someone who has essentially finished their meal, but uses a toothpick to pick tiny bits of food out of the corners of the tray. The phrase has a negative nuance and is commonly invoked to criticize someone’s behavior or attitude.
We begin at the end with the verb 突く (tsutsuku), “to poke,” in conclusive form. The particle を (wo) marks the noun 隅 (sumi), “corner,” “recess,” as the object of the verb. Meanwhile, the associative particle の (no) associates 隅 with, and allows it to be modified by, the noun 重箱 (juubako), a “multi-tiered food box.”
Bear in mind that while the English “corner” can refer to a convex or a concave angle, 隅 is only concave; it refers to the “inside” part of a corner. The “outside” (convex) part is a 角 (kado), as seen in terms like 三角 (sankaku), “triangle.” Naturally, replacing 隅 with 角 is an error.
That said, this saying does allow for quite a few variations. Tsutsuku may be written in kana as つつく. It may be replaced by ほじくる (hojikuru), “to dig out,” “to pick [one’s nose, teeth, etc.].” The specificity of の隅 may be elided, or specificity may be added by saying that the picking is done 楊枝で (youji de), “with a toothpick”; this may be added at the beginning of the sentence, or just before the verb.
重箱 is a fascinating word. It’s a rare example of a compound in which one character uses the Chinese-origin on reading while the other uses the Japanese-origin kun reading (in this case, respectively juu for 重 and hako for 箱). In fact, juubako is the ur-example of an on/kun combination, while the kun/on equivalent is 湯桶 (yutou), a “container for hot liquids” that might be used to serve drinks, or for washing in a traditional bath or onsen. Beyond this, while 重 is commonly translated as “heavy,” here we see it refer to something that is “piled up” or “layered”; cf. 二重 (futae), “twofold.”
(“Oniichan ga juku ni kayoi hajimete kara, nandemo kandemo juubaku no sumi wo tsutsuku you na iikata wo suru you ni natta. Nande darou, chotto iya na kibun.”)
[“Ever since my big brother started going to cram school, he’s been nitpicking every little thing, no matter what we’re talking about. I wonder why; it’s kind of gross.”]