Meaning of rock alum in English:

rock alum

Pronunciation /ˈrɒk ˌaləm/

See synonyms for 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.