A stupid person.
"Stupid person," by 1951, American English, from dumb. The Disney musical cartoon is from 1941; in it the elephant's name is a mocking nickname based on Jumbo.
A fool, a twit.
A small or puny person; one regarded as insignificant, contemptible.
A person who can be bullied playfully, or easily teased. Sometimes used as a pet-name (often for a younger sibling).