Putting on an innocent act with

猫(ねこ)を被(かぶ)る refers to hiding one’s true personality and acting gentle and docile. It can also mean pretending not to know something, similar to “play innocent” in English.

Literal Meaning

The expression translates word for word as “wear a cat”. The verb 被(かぶ)る means to put something on the head, creating the image of wearing a cat’s quiet appearance over one’s true character.

Usage Notes

This idiom is used when someone shows unusual restraint around anyone they want to impress.

猫(ねこ)を被(かぶ)っている describes the ongoing act, and can also be used to refer to oneself. 猫(ねこ)を被(かぶ)っていた looks back on a period when the person kept up the appearance. The noun 猫(ねこ)被(かぶ)り labels a person who puts on this kind of act.

Dictionary Definition

本性(ほんしょう)をかくしておとなしそうに見(み)せる。また、知(し)っていながら知(し)らないふりをする。

To hide one's true nature and make oneself look meek. Also, to pretend not to know something despite knowing it.

精選版 日本国語大辞典

Examples

Being innocent at school with 猫を被る

Manga panel from ぼくの魔なむすめ showing example of Being innocent at school with 猫を被る.
ぼくの魔なむすめ » Volume 1 » Page 138

Sami and her father go to the park to play. There, they see Sami’s teacher. The teacher is surprised to see Sami’s rambunctious behavior, as it’s different from the Sami she sees at school.

先生(せんせい):
「学校(がっこう)でのサミさんはすごくかわいい爽(さわ)やかな表情(ひょうじょう)をしています!」
“At school, Sami has such a cute and delightful expression!”
パパ:
「ちょっと猫(ねこ)かぶってるんだなぁ…」
“She's just putting on a front, isn't she...”