(Told by an idiot)

大山鳴動して鼠一匹
(Taizan meidou shite nezumi ippiki; “The mountain trembles; a single mouse.”)

Definition:

Terrible harbingers and portents without follow-through. Much ado, after which, nothing. Especially used in reference to empty threats. Like a large mountain rumbling and shaking as if about to erupt, only for it to turn out that all the noise was echoes raised by a lone mouse.

Breakdown:

We begin with compound noun 大山 (taizan), “large mountain.” Any particle marking this noun is elided, but it’s followed by noun 鳴動 (meidou), “rumbling,” made into a verb by the addition of する (suru), “to do.” The verb is in conjunctive form, and is followed by noun-number-counter 鼠一匹 (nezumi ippiki), “one mouse.”

Notes:

Taizan may be written with as 太山 without any change in meaning or pronunciation. It’s also sometimes mistaken for 泰山 – China’s famous Mount Tai, and this writing is actually so common and established that it’s considered acceptable. However, the point of entry into Japanese is apparently from the writings of the Roman poet Horace, as “Parturiunt montes, nascitur ridiculus mus.” (“The mountain was pregnant, and birthed a ridiculous mouse.”) And Horace himself was drawing on earlier traditions, perhaps back to Aesop’s Fables.

Example sentence:

「あの教頭先生？いつも怒鳴りつけてくるからやっぱり怖く見えるけど、怒鳴りつける以外は一切行動しないよ。大山鳴動して鼠一匹ってことさ。」

(“Ano kyoutou sensei? Itsumo donari tsukete kuru kara yappari kowaku mieru kedo, donari tsukeru igai wa issai koudou shinai yo. Taizan meidou shite nezumi ippiki tte koto sa.”)

[“The vice-principal? He always yells at people, so yeah, he looks scary, but other than yelling he does absolutely nothing. He’s all bark and no bite.”]