What does alum mean?
The definition of alum
Al”um, n.
Etym: [OE. alum, alom, OF. alum, F. alun, fr. L. alumen alum.] (Chem.)

A double sulphate formed of aluminium and some other element (esp. an alkali metal) or of aluminium. It has twenty-four molecules of water of crystallization.

Note: Common alum is the double sulphate of aluminium and potassium. It is white, transparent, very astringent, and crystallizes easily in octahedrons. The term is extended so as to include other double sulphates similar to alum in formula.

Al”um, v. t.
To steep in, or otherwise impregnate with, a solution of alum; to treat with alum. Ure.