The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            By storm and onslaught to proceed.    --Hudibras.
      [1913 Webster]

   2. A bloody fray or battle. [Scot.] --Jamieson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, onslaught,
       onset, onrush]
    3: the rapid and continuous delivery of linguistic communication
       (spoken or written); "a barrage of questions"; "a bombardment
       of mail complaining about his mistake" [syn: barrage,
       bombardment, outpouring, onslaught]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "onslaught":
   abuse, aggravated assault, aggression, amphibious attack, appulse,
   armed assault, assailing, assailment, assault, attack,
   banzai attack, battering, berating, bitter words, blackening,
   blitz, blitzkrieg, breakthrough, brunt, bulldozing, bulling, bump,
   butchery, cannon, carambole, carom, charge, clash, collision,
   concussion, contumely, counterattack, counteroffensive,
   coup de main, crack-up, crash, crippling attack, crump, crunch,
   dead set at, descent on, diatribe, disorderliness, diversion,
   diversionary attack, drive, encounter, execration, flank attack,
   forcible seizure, frontal attack, gas attack, hammering,
   hard words, head-on attack, impact, impingement, infiltration,
   invective, jawing, jeremiad, killing, laying waste,
   lightning attack, lightning war, looting, mass attack, massacre,
   mauling, meeting, megadeath, mugging, obstreperousness, offense,
   offensive, onset, overkill, panzer warfare, percussion, philippic,
   pillaging, push, ramming, rape, rating, revilement, riot, rioting,
   run against, run at, rush, sacking, sally, screed, shock,
   shock tactics, sideswipe, slaughter, sledgehammering, smash,
   smash-up, smashing, sortie, sowing with salt, strike, thrusting,
   tirade, tongue-lashing, unprovoked assault, unruliness,
   vilification, violation, vituperation, whomp