Source : Webster's Revised Unabridged Dictionary (1913)

Onslaught \On"slaught`\, n. [OE. on on + slaught, slaht,
   slaughter. See {Slaughter}.]
   1. An attack; an onset; esp., a furious or murderous attack
      or assault.

            By storm and onslaught to proceed.    --Hudibras.

   2. A bloody fray or battle. [Scot.] --Jamieson.

Source : WordNet®

onslaught
     n 1: a sudden and severe onset of trouble
     2: (military) an offensive against an enemy (using weapons);
        "the attack began at dawn" [syn: {attack}, {onset}, {onrush}]
     3: the rapid and continuous delivery of linguistic
        communication (spoken or written) [syn: {barrage}, {outpouring}]