(Yes, 取 supposedly means “take” because it refers to taking a defeated enemy’s ear as a trophy in battle. So… appropriate?)

鬼の首を取ったよう
(Oni no kubi wo totta you; “As if having taken an oni’s head”)

Definition:

Triumphant or boastful, especially about something that’s not really that big a deal. As proud as if you’d slain a demon or defeated a monster – without having actually accomplished much.

Breakdown:

This adjectival noun phrase begins with the noun 鬼 (oni), a mythical monster somewhere between “really strong person” or “ogre,” all the way up to “demon from one of the Buddhist hells.” A common enemy in old folk-tales. The associative particle の () in its possessive function connects the oni to its 首 (kubi), “neck,” or by extension, “head.” This entire noun phrase is marked by the particle を (wo) as the object of the verb 取る (toru), “to take,” in past tense. It precedes and attaches to the noun よう (you, pronounced to rhyme with “toe”), “appearance,” “form.” Like all adjectival nouns, it will tend to be followed by な (na) and then a noun phrase, に (ni) and then a verb phrase, or by a copula.

Notes:

This phrase apparently comes to us from an early 1711 CE ukiyozoushi called 傾城禁短気 (Keiseikin tanki).

Example sentence:

男の子は鬼の首を取ったような満面の笑みを浮かばせ、慌てて逃げ出そうと全力を尽くすバッタを握っていた。

(Otoko no ko wa oni no kubi wo totta you na manmen no emi wo ukabase, awatete nigedasou to zenryoku wo tsukusu batta wo nigitteita.)

[Clutching a grasshopper that was trying with all its might to escape, the boy was smiling as proudly as if he’d slain an ogre.]