Charlie Chaplin was called the tramp because that was the name of a character he created in 1913 when he joined the Keystone motion picture company. The character had a short mustache and he wore a suit with baggy trousers and he carried a top hat and a cane.
His character tried to act like a gentleman in his fancy clothes, top hat and cane, but his baggy pants and silly actions made him look like a vagrant or a tramp. His slapstick activities made people laugh at his antics. You almost had to feel sorry for him but at the same time you could laugh at him.