as stubborn as a mule = very stubborn
There’s no point persuading him to join us: he’s as stubborn as a mule.
to flog a dead horse = to waste time trying to do something that will not succeed
We should admit our plan is a failure. All we are doing now is flogging a dead horse.
to have kittens = to get anxious
She has kittens whenever I mention her losing her job.
like water off a duck’s back = easily, without any apparent effect
Don’t waste your time telling him off. It’s like water off a duck’s back.
to let sleeping dogs lie = to not talk about things which have caused problems in the past, or to not try to change a situation because you might cause problems
We’d better not raise this question at the meeting. Let sleeping dogs lie.