衆口金を鑠かす
(Shuukou kin wo tokasu;
“The mouths of the masses melt metal.”)

Definition:

Rumor and slander tend to get worse as they spread. The power of many people all talking about something is to be feared, because even correct information has a tendency to get altered or distorted. The phrase invokes the terrifying power of a smelter, which can reduce even strong metals to liquid.

Breakdown:

We begin with compound noun 衆口 (shuukou), literally “the mouth of the masses,” i.e. something that many people are talking about. A possible clarifying particle is elided, and we move straight to a verb phrase. This is the verb 鑠く (toku), “to melt,” with the particle を (wo) marking the noun 金 (kin), “gold,” or by extension “metal,” as its direct object. The verb appears in imperfective form, with causative suffix す (su), in sentence-final form.

Notes:

This saying comes to us from the Discourses of the States (國語, in current Japanese 国語, Kokugo), a 4th century BCE compilation of speeches (“discourses”) attributed to various famous historical figures.

The character 鑠 is no longer in common usage and may be hard to produce by typing. It looks like it can be replaced by near-synonymous homophone 爍 (near-synonymous because 鑠 carries a secondary meaning of “charmed, captivated,” while 爍 carries a secondary meaning of “to shine”). None of my sources use 鎔 or 熔, the moderns character for “melt [metal].”

Example sentence:

鍛治君は公職に就いてからすぐに噂の恐ろしさを知り、衆口には金を鑠かすほどの力があると悟りました。

(Kaji-kun wa koushoku ni tsuite kara sugu ni uwasa no osoroshisa wo shiri, shuukou ni wa kin wo tokasu hodo no chikara ga aru to satorimashita.)

[After taking public office, Kaji quickly became acquainted with how frightening rumor is, and came to realize that public discussion can have the power of a blast furnace.]