斜に構える
(Sha ni kamaeru; “take a diagonal stance”)

Definition:

To behave as if the situation isn’t serious; to adopt a (mockingly) aloof attitude, especially toward an opponent or problem. To view things in an ironic or cynical light.

Breakdown:

Technically this verb phrase, by virtue of having a verb, could serve as a complete sentence… it probably won’t, though.

We have a noun, 斜 (sha), “diagonal,” and a verb, 構える (kamaeru), “to set (something) up.” They’re connected by the locational/directional particle に (ni).

Notes:

This idiom comes from kendo (Japanese swordplay), describing a stance in which the sword is held at a downward angle, below waist level. Unlike many stances, this makes it difficult both to threaten a quick and deadly strike against one’s opponent and to defend oneself readily. Thus, the implication is that one isn’t taking one’s opponent seriously as a threat.

Example sentence:

斜に構えて真剣勝負をしてくれない相手は速攻で倒せるほどの力を持つ選手であった。

(“Sha ni kamaete shinken shoubu wo shite kurenai aite wa sokkou de taoseru hodo no chikara wo motsu senshu de atta.”)

[“S/he was an athlete who was strong enough to immediately knock down any opponent who adopted a dismissive attitude instead of fighting seriously.”]