According to one theory, generally viewed as a bit of urban
folklore, this term originated with a cowardly newspaperman and an uncomprehending typesetter.
Allegedly, there was a gang leader in San Francisco named Muldoon sometime around the turn of the century. Muldoon was so feared that a newspaper reporter who did a story on him attempted to disguise the identity of his subject by spelling the name backward as "Noodlum". The typesetter misread it as "hoodlum" and a new word was coined.