What was Big Willy Shakes going for?
King John wants some elbow room, or space to move around his elbows. This phrase is literal. It means to clear the way, make room, give a guy some space.
But it's not for his body. He wants some room for his soul. Shakespeare takes a well-known saying in his day and applies it to the soul instead of the body. And we're pretty sure everyone knows what he means here. He wants his conscience to have some room to breathe. Maybe because he's gone whacking people in the name of power for so long, his guilty conscience is kicking in.
Shakespeare loves to take a cuckoo character and give them lines that make the most sense. It looks like that's what's happening here. We fully expect King John to have lost his marbles, and well, he totally has. But, underneath that, he speaks with clarity about how he feels about everything he's done. We hate to be the ones to say it, but if he needs some elbow room for his soul, it might not be such a good thing.