The Collaborative International Dictionary of English v.0.48:

Brimstone \Brim"stone\, n. [OE. brimston, bremston, bernston,
   brenston; cf. Icel. brennistein. See Burn, v. t., and
   Stone.]
   Sulphur; See Sulphur.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Brimstone \Brim"stone\, a.
   Made of, or pertaining to, brimstone; as, brimstone matches.
   [1913 Webster]

         From his brimstone bed at break of day
         A-walking the devil has gone.            --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brimstone
    n 1: an old name for sulfur [syn: brimstone, native sulfur,
         native sulphur]

Easton's 1897 Bible Dictionary:

Brimstone
   an inflammable mineral substance found in quantities on the
   shores of the Dead Sea. The cities of the plain were destroyed
   by a rain of fire and brimstone (Gen. 19:24, 25). In Isa. 34:9
   allusion is made to the destruction of these cities. This word
   figuratively denotes destruction or punishment (Job 18:15; Isa.
   30:33; 34:9; Ps. 11:6; Ezek. 38:22). It is used to express the
   idea of excruciating torment in Rev. 14:10; 19:20; 20:10.