The 'dum-dum' was a British military bullet developed for use in India - at the Dum-Dum Arsenal - on the North West Frontier in the late 1890s.
The dum-dum comprised a jacketed .303 bullet with the jacket nose open to expose its lead core. The aim was to improve the bullet's effectiveness by increasing its expansion upon impact.
The phrase 'dum-dum' was later taken to include any soft-nosed or hollow pointed bullet. The Hague Convention of 1899 outlawed the use of dum-dum bullets during warfare.