“He tried to say he didn’t do it, but they caught him with his hand in the cookie jar.” 「やってないと言い張ったがクッキー・ジャーに手を突っ込んだまま見つかってしまった。」


Imagine you are a child asking your mother if you can have a cookie. She says no, but you go to the kitchen and take a cookie from the cookie jar anyway. A child trying to get a cookie from a jar on a high shelf is a well-known image in the USA, used as a metaphor for any kind of crime.