Meaning

noun a soft-nosed small-arms bullet that expands when it hits a target and causes a gaping wound

originFrom Dum Dum, a city in India where the type of bullet was developed, from Hindi दम दम (dam dam) (Bengali দমদম (domodom)), from Hindi दमदमा (damadmā, “tenaille, a raised mound or battery”).

Etymology from Wiktionary, CC BY-SA 3.0