空き樽は音が高い
(Akidaru wa oto ga takai; “An empty barrel makes a loud noise”)

Definition:

If you strike an empty barrel, the sound it produces is high-pitched, sharp, and loud instead of the muted thump you’d get from a full one. Similarly, a human who isn’t filled with any meaningful amount of knowledge tends to produce lots of self-important talk despite their hollowness. Shallow waters run loud; empty vessels make the most noise.

Breakdown:

We begin with the verb 空く (aku), “to be open” or in this case, “to be empty,” in conjunctive form as 空き (aki) – which is odd, and we’ll look at in in depth later. In any case it seems to attach to, and modify, the noun 樽 (taru), “cask,” “barrel,” etc. The particle は (wa) marks this noun phrase as the topic of discussion. The following comment begins with noun 音 (oto), “sound,” marked as the subject of its clause by the particle が (ga), and finally as the predicate to this subject we get the adjective 高い (takai) in conclusive form.

Notes:

It’s okay to write akidaru in more compact form as 空樽 without any change in meaning or pronunciation. Similarly, the final noun and adjective may be rendered in a more old-fashioned form as 音高し (oto takashi), again without changing the meaning.

As mentioned above, the fact that we use akidaru strikes me as odd. The conjunctive form would normally be followed by a verb phrase or a full clause or sentence, not a noun, after all. And the prenominal form of the verb is nowhere to be sen. Yet there are many such phrases, including 空き缶 (akikan), an empty can, 空き殻 (akigara), an empty shell, and so on. My favorite of these is 空き店, which is hilarious because the final character means “store,” as in bookstore – but the pronunciation is akidana, where tana normally means a shelf – and an 空き店’s primary usage is referring to an empty house. Figure that one out!*

Anyway! Two reasons come to mind why 空き might be the way it is:

1. As we’ve seen before every now and then, the conjunctive form of a verb can act as a noun. Perhaps this 空き樽 is actually a compound noun.

2. Changing to ki in prenominal form would actually be appropriate for an adjective. Perhaps aki is simply being treated as an adjective; this would make the base/conclusive form into 空し (ashi).

There’s just one problem with that second theory: I can’t find any sources for ashi as an adjective. There is 空し, pronounced munashi, and there’s 空 (kara), which is a noun that’s almost always used in an adjectival sort of way, but nothing that would grammatically become aki on that front.

In brief, the most likely grammar for 空き樽 is a compound noun, where the first noun is a frozen conjugated form of a verb, which is being used in an adjectival way to modify its fellow noun.

*: This is actually because tana, written as 店, can refer to a merchant’s (rented) living quarters as well as to the shop proper, presumably from back in the day when a lot of people had ancestral homes out in a village somewhere but worked in the city, and found it efficient to live where they worked.

Example sentence:

「隣の稲田さんは、人としてはいいんだけど、自分の声に聞き惚れていて、音の高い空き樽になるきらいもたまにあるんだよ」

(“Tonari no Inada-san wa, hito to shite wa ii nda kedo, jibun no koe ni kikihoreteite, oto no takai akidaru ni naru kirai mo tama ni aru nda yo.”)

[“My neighbor Inada is a good person, but they really like the sound of their own voice, and they have this unpleasant tendency to talk a lot even when they know very little.”]