Definition
To have a very thorough and detailed understanding of something; to be extremely familiar with something.
對某事物瞭如指掌；對某事非常熟悉。
I know the menu at this restaurant like the back of my hand, I can recommend some great dishes to you.
我對這家餐廳的菜單瞭如指掌，我可以推薦一些很棒的菜給你。
He knows the streets of this city like the back of his hand, he never needs to look at a map.
他對這個城市的街道瞭如指掌，他從來不需要看地圖。
She knows the details of this project like the back of her hand, she is in charge of overseeing all the processes.
她對這個項目的細節瞭如指掌，她負責監督所有流程。