Or just about anything at all before a goat

鰹節を猫に預ける
(Katsuobushi wo neko ni azukeru;
“Entrusting katsuo shavings to a cat”)

Definition:

A dangerous situation where one can’t afford to let one’s guard down. Risking a loss by presenting someone with an overpowering temptation. Setting a fox to guard the henhouse. Like placing your katsuo shavings – a tasty delicacy – next to a cat, who is almost certain to immediately try to eat them.

Breakdown:

We begin at the end with the verb 預ける (azukeru), to leave something in someone’s care; to entrust something to someone for safekeeping or proper use. This verb appears in conclusive form. Before it we find the directional particle に (ni), “to,” marking the indirect object of the verb. This object is the noun 猫 (neko), “cat.” And preceding this we find the indirect-object particle を (wo), marking the compound noun 鰹節 (katsuobushi). This refers to thin flakes or slices of dried meat from a skipjack tuna, a.k.a. the “arctic bonito,” which is closely related to – but not quite the same as – the “true” bonito genera, and commonly translated as “bonito.”

Notes:

鰹節 may also be read as katsubushi (dropping the o), although this is nonstandard.

This phrase may be shortened and flipped to 猫に鰹節 (neko ni katsuobushi) without any change in meaning. This is just the first in a panoply of synonymous phrases: one may leave the cat in charge of dried skipjack (in chunk form), or just a fish; a fox in charge of adzuki-rice; a thief in charge of a key, or in charge of guarding the storehouse; or even a goldfish in charge of mosquito larvae (!).

Alert readers may remember phrases such as 猫に小判 and wonder if this phrase is an antonym. It doesn’t seem to be counted as one, though – and this makes sense. Despite the syntactic similarity, the meanings aren’t actually in opposition.

One of my sources attributes this saying to a work titled 『根無草』 (Nenashigusa), although further details are scant.

Example sentence:

「妻の誕生日ケーキをちゃぶ台に置いて、ろうそくを取りに行ったら、娘が手づかみでケーキを食べていた。残念なことにケーキは二歳児の手の届く位置にあったのだ。流石に、そんな鰹節を猫に預けたような油断をしたのは大失敗だった」

(“Tsuma no tanjoubi keeki wo chabudai ni oite, rousoku wo tori ni ittara, musume ga tedzukami de keeki wo tabeteita. Zannen na koto ni keeki wa nisaiji no te no todoku ichi ni atta no da. Sasuga ni, sonna katsuobushi wo neko ni azuketa you na yudan wo shita no wa daishippai datta.”)

[“I put my wife’s birthday cake on the coffee table and went to get candles, and then our daughter was eating it by the handful. Unfortunately, the cake was in a place where a two-year-old could reach it. And yeah, dropping my guard and creating such a tempting target was a really big mistake on my part.”]