Originally nicknamed Chicko because in those days women were referred to as chicks and the guys who chased them as chicken chasers. A typesetter accidentally dropped the k in his name and it became Chico, but it was still pronounced as if it were Chicko. Chico developed the "Italian" accent he used to convince some roving bullies that he was Italian, not Jewish. Stereotyped ethnic characters were common with Vaudeville comedians, and all the Marx brothers sometimes performed "dialect characters" early in their careers, but Chico was the only one to continue this.
Chico was a talented pianist. As a young boy, he would get jobs playing piano to earn money for the Marx family. Sometimes Chico would even get work playing in two places at the same time. He would acquire the job with his piano playing skills, work for a few nights, and then substitute Harpo on one of the jobs. The two brothers looked so much alike, no one could tell the difference. Harpo however could only play a few tunes on the piano, which often would get both brothers fired.
Chico became manager of the Marx Brothers after their mother, Minnie, died. As manager he cut a deal to get the Marx Brothers a percentage of a film's gross receipts - the first of its kind in Hollywood.
Chico Marx had a lifelong gambling habit, which usually kept him short of funds, and which compelled him to continue in show business long after his brothers had retired in comfort from their Hollywood income. (Groucho continued to host the long-running televison show "You Bet Your Life" out of his love of being before an audience rather than any financial need.) The last two Marx brothers movies were made for Chico's benefit; the other brothers twice returned to the screen to bail Chico out of debt. He had a reputation as a world-class pinochle player. It is said he would throw away good cards (with the knowledge of spectators) to make the play "more interesting".