It cost me an arm and a leg to take my trip to Australia.

an arm and a leg: Very expensive

I was over the moon when he asked me to marry him.

over the moon: Extremely pleased or happy

You are taking your IELTS test next week?? Aren’t you jumping the gun? You’ve only just started studying.

jumping the gun: Doing or starting something too early

He comes round to see me once in a blue moon.

once in a blue moon: Happening very rarely

He’s got a chip on his shoulder.

a chip on his shoulder: Feeling inferior or having a grievance about something