1660s, cast iron, from iron (n.) + cast (adj.) "made by melting and being left to harden in a mold" (1530s), past-participle adjective from cast (v.) in its sense "to throw something (in a particular way)," c. 1300, especially "form metal into a shape by pouring it molten" (1510s). From 1690s as an adjective, "made of cast-iron;" figurative sense of "inflexible, unyielding" is from 1830.
type of metal-cased bullet which expands on impact, 1897, named for Dum-Dum arsenal in Bengal. British army soldiers made them to use against fanatical charges by tribesmen. Outlawed by international declaration, 1899. The place name is literally "hill, mound, battery," cognate with Persian damdama.
It was to stop these fanatics [Ghazi] — and that firstclass fighting man Fuzzy-Wuzzy, of the Soudan and of Somaliland—that the thing known as the Dum-Dum bullet was invented. No ordinary bullet, unless it hits them in a vital part or breaks a leg, will be sufficient to put the brake on these magnificently brave people. ["For Foreign Service: Hints on Soldiering in the Shiny East," London, 1915]
Pile on the brown man's burden
And if ye rouse his hate,
Meet his old fashioned reasons
With Maxims up-to-date;
With shells and dum dum bullets
A hundred times make plain,
The brown man's loss must ever
Imply the white man's gain.
[Henry Labouchère, from "The Brown Man's Burden," 1899]