大根を正宗で切る
(Daikon wo Masamune de kiru; “To cut a daikon with a Masamune”)

Definition:

Overkill. Bringing force, talent, or material to a job or problem far in excess of what is actually needed; often, assigning a task to someone who is massively overqualified for it. Like using a legendary blade to chop vegetables.

Breakdown:

We begin with the noun 大根 (daikon), a vegetable related to radishes whose name literally means “big root.” The particle を (wo) marks this noun as the object of the verb at the end of the phrase, 切る (kiru), “to cut.” And in the middle we have the particle で (de) marking the means by which the verb is performed; in this case, proper noun 正宗 (Masamune), a Kamakura-era swordsmith whose name is used metonymically to refer to one of the blades he made.

Notes:

Gorō Masamune is a partially mythologized figure who is generally held to be the country’s greatest swordsmith of all time. Using any sword to cut vegetables would be ridiculous enough as it is; all the more so if it’s a masterpiece by a famous master.

Writing Masa with homophone 政 is an error; that’s just not how the guy’s name is spelled. On the other hand, it’s perfectly acceptable to write kiru as 斬る, or rearrange things to get 正宗の刀で大根切る (Masamune no katana de daikon kiru).

Example sentence:

「課長、すみませんが、高田さんにお茶汲みだなんてそんなどうでもいいことをさせるのは大根を正宗で切るほどもったいないんじゃないですか？」

(“Kachou, sumimasen ga, Takada-san ni ochakumi da nante sonna dou de mo ii koto wo saseru no wa daikon wo Masamune de kiru hodo mottainai nja nai desu ka?”)

[“Chief, excuse me, but isn’t having Takada do meaningless little jobs like tea duty just a waste? It’s like using a Masamune to chop daikon.”]