The Collaborative International Dictionary of English v.0.48:

Onset \On"set`\, v. t.
   1. To assault; to set upon. [Obs.]
      [1913 Webster]

   2. To set about; to begin. [Obs.] --Carew.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Onset \On"set`\, n. [On + set.]
   1. A rushing or setting upon; an attack; an assault; a
      storming; especially, the assault of an army. --Milton.
      [1913 Webster]

            The onset and retire
            Of both your armies.                  --Shak.
      [1913 Webster]

            Who on that day the word of onset gave.
                                                  --Wordsworth.
      [1913 Webster]

   2. A setting about; a beginning; -- used especially of
      diseases or pathological symptoms. --Shak.
      [1913 Webster +PJC]

            There is surely no greater wisdom than well to time
            the beginnings and onsets of things.  --Bacon.
      [1913 Webster]

   3. Anything set on, or added, as an ornament or as a useful
      appendage. [Obs.] --Johnson.
      [1913 Webster]

U.S. Gazetteer (1990):

Onset, MA (CDP, FIPS 51160)
  Location: 41.74758 N, 70.66758 W
  Population (1990): 1461 (1189 housing units)
  Area: 2.8 sq km (land), 0.6 sq km (water)
  Zip code(s): 02532

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "onset":
   A, aggravated assault, aggression, albertype, alpha,
   amphibious attack, appearance, armed assault, assailing,
   assailment, assault, attack, banzai attack, beginning, birth,
   blast-off, blitz, blitzkrieg, book printing, breakthrough, charge,
   chromotypography, chromotypy, chromoxylography, collotype,
   color printing, commencement, counterattack, counteroffensive,
   coup de main, creation, crippling attack, cutting edge, dawn,
   dawning, dead set at, debut, descent on, diversion,
   diversionary attack, drive, edge, electronography,
   electrostatic printing, establishment, flank attack, flying start,
   foundation, fresh start, frontal attack, gas attack, genesis,
   graphic arts, gravure, halftone engraving, head-on attack,
   history of printing, hit, inauguration, inception, infiltration,
   initiation, institution, job printing, jump-off, kick-off,
   leading edge, letterpress, letterpress photoengraving,
   lightning attack, lightning war, line engraving, lithography,
   lithogravure, lithophotogravure, mass attack, megadeath,
   mimeograph, mugging, new departure, offense, offensive, offset,
   offset lithography, oncoming, onrush, onslaught, opening, origin,
   origination, outbreak, outset, outstart, overkill,
   palaeotypography, panzer warfare, photo-offset,
   photochemical process, photoengraving, photogelatin process,
   photographic reproduction, photography, photolithography,
   phototypography, phototypy, photozincography,
   planographic printing, planography, printing, printmaking,
   publication, publishing, push, raid, relief printing,
   rotary photogravure, rotogravure, run against, run at,
   running start, rush, sally, send-off, setout, setting in motion,
   setting-up, sheetwork, shock tactics, sortie, square one, start,
   start-off, starting point, stencil, storming, strike, take-off,
   three-color printing, two-color printing, typography,
   typolithography, unprovoked assault, wood-block printing,
   xerography, xeroprinting, xylotypography, zincography