Why is jelly role morton's nickname jelly roll?

At the age of fourteen, Morton began working as a piano player in a brothel (or, as it was referred to then, a sporting house). While working there, he was living with his religious, church-going great-grandmother; he had her convinced that he worked as a night watchman in a barrel factory.

In that atmosphere, he often sang smutty lyrics; he took the nickname "Jelly Roll", which was black slang for female genitalia.