Definition of rock alum in US English:

rock alum

noun

historical
  • Crystalline alum of high purity, especially that prepared from Italian alunite.

Origin

Mid 17th century. From rock + alum, after French alun de roche (1374 in Middle French; compare Anglo-Norman alome de roche, alum Roche, and Middle French (Tournai) alun en roque) or Italian allume di rocca. Compare post-classical Latin alumen de roca, alumen de rocco, Spanish alumbre de roque, and also Spanish piedra alumbre, Portuguese pedra-ume, in same sense, which show substitution of the respective Spanish and Portuguese reflexes of classical Latin petra rock, stone.

Pronunciation

rock alum

/ˈrɒk ˌaləm/