What does "put one's back into something" mean?

Native speaker’s answer
Rebecca
"Put your back into it" is a slang expression meaning to put a lot of effort (usually physical) into doing something. Using your back is similar to using your whole body/using a lot of effort. Paddington is telling himself to "put your back into it" to encourage himself to use his whole body/put in a lot of effort to lift the giant mountain of oranges up. Ex: Put your back into it! This box is heavy. Ex: You have to put your back into it if you want to move this thing. It's super heavy.