Source : Webster's Revised Unabridged Dictionary (1913)

Foam \Foam\, v.i. [imp. & p. p. {Foamed}; p. pr. & vb. n.
   {Foaming}.] [AS. f?man. See {Foam}, n.]
   1. To gather foam; to froth; as, the billows foam.

            He foameth, and gnasheth with his teeth. --Mark ix.
                                                  18.

   2. To form foam, or become filled with foam; -- said of a
      steam boiler when the water is unduly agitated and frothy,
      as because of chemical action.