Highwaymen are criminals that assault young adventurers under orders of Dirk Turnip, their leader. They used to be led by Robin, but his crazy ways made them leave. As the highwayman Jeremy Louis “The Lesser of Two Evils” Shawaver suggests, however, Dirk is not much better than Robin. Highwaymen will attack players up to level 10 and may prove a challenge to new players who are just passing by. They always will attack while saying "Stand and deliver!".
They are not aggressive to players who have a combat level of 33 or above. It is possible that some highwaymen were created to stop bots and macroers. Highwaymen can be deadly to new players trying to get to Falador.
"Stand and Deliver!" is a reference to a phrase most often used in the 17th Century where highwaymen would use the phrase while robbing travelers, which meant to stand still and hand over their money and valuables.
In the 29th Postbag from the Hedge one of the highwaymen revealed that his name is Dirk Turnip and that he is "NOT a dandy highwayman". His name is probably a reference to famous highwayman Dick Turpin, and the denial a reference to the song Stand and Deliver.