Use Of English Words To Mean Something Else...

I think Higher Ed is the place for this. It's for a person who studies language.

What is it called when you use a phrase to mean something totally different than its original meaning? The meaning is similar, but that's obviously not what you mean. For example in Mark Anthony's eulogy for Caesar in the play Julius Caesar he says:

Bear with me;
My heart is in the coffin there with Caesar.

Caesar's body was not in a box, or coffin. It was on the floor wrapped in his mantle. Or sometimes people in the US might say a dog is going to the bathroom on their lawn, even though he is nowhere near a restroom. What is that called? It's name in grammar I mean.