If you know something really well, you know it 'like the back of your hand'

• I know my way around New York like the back of my hand.

If criticism has no effect on somebody, it is 'like water off a duck's back'.

• I told her yet again about being late for meetings but it's like water off a duck's back with her.

If a place is very isolated geographically, it is 'in the back of beyond'.

• They set up their new factory in the back of beyond. There is no airport for two hundred miles.