焼け石に水
(Yakeishi ni mizu; “Water on a hot stone”)

Definition:

A situation where you shouldn’t expect much. A drop in the bucket. A negligible amount of aid or effort, like a little splash of water against a hot stone, which quickly turns to steam and disappears without cooling the stone noticeably.

Breakdown:

We begin with the verb 焼く (yaku), “to burn,” “to heat,” in prenominal form and prefixing the noun 石 (ishi), “stone.” The heated stone is marked by the directional/location particle に (ni) as the target of the noun 水 (mizu), “water.” One can imagine that a verb (such as 掛ける, kakeru, “to throw something against something”) follows, but any verb is elided in the idiom itself.

Notes:

This phrase is apparently related to a 焼き石 (yakiishi), a stone heated and placed inside a bowl of soup. Apparently one of these could be used to bring a meal to boiling without any need for direct heating from a fire, so it stands to reason that a little splash wouldn’t do much to cool one off.

The origins of this phrase are unclear, but possible sources I’ve seen cited include two 17th-century poetic treatises, the 世話焼草 (Sewayakigusa), and the 毛吹草 (Kefukigusa).

Example sentence:

「偉い弁護士を雇ったが、焼け石に水だった。むしろ逆効果だったような気がする」

(“Erai bengoshi wo yatotta ga, yakeishi ni mizu datta. Mushiro gyakukouka datta you na ki ga suru.”)

[“I hired a hot-shot lawyer, but it didn’t help at all. On the contrary, I think he made things worse.”]