That joke (or at least a version of it) has been around since 1939, and is from the script of the Laurel and Hardy film "A Chump at Oxford." as written by Stan Laurel

Lord Paddington : Fatty, I've noticed lately that you're getting terribly sloven.

Ollie : How do you mean, sir?

Lord Paddington : Well, you don't seem to have the dignity becoming of a lackey. No poise. You don't walk right or something. Let me see, pull in your stomach. That's better. Now, now throw your shoulders back. That's fine. Now, chin up. Chin up. No no no, no. Both of them!