In the eighteenth century, poker cards were called broads because they were wider than those used for other card games.
Around 1912, because they resembled poker cards, tickets of admission, meal tickets, and transit tickets were being called “broads.”
By 1914, because they were a different kind of “meal ticket,” pimps began calling their prostitutes “broads.”
Soon the term entered the underworld and was eventually picked up by entertainers.