a bad mood, especially one in which a person will not do as they are asked and is unpleasant to other people

Don't go in unless you have to - she's in a (real) strop. 若非必要就别进去——她正在气头上。

He's impossible to talk to when he's having one of his strops.

When she didn't get what she wanted she threw a strop.

to be in a bad mood

Why have you got such a strop on - what's happened? 你为什么发这么大的脾气——发生什么事了？