A round mass of snow pressed or roller together, or anything resembling such a mass. [1913 Webster]
The Guelder-rose. [1913 Webster]
a shrub of the genus Viburnum, having large clusters of white flowers. [PJC]

Snowball bush or Snowball tree (Bot.), the Guelder-rose. -- a snowball's chance in hell, [Colloq.] no chance; an infinitesimal chance.

snowball, v. t.

To pelt with snowballs; to throw snowballs at. [1913 Webster]

snowball, v. i.

To throw snowballs. [1913 Webster]
To increase in magnitude at an accelerating rate, achieving large proportions; -- by analogy with a snowball rolling down a steep hill, causing a large snow slide. [PJC]

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OXFORD DICTIONARY

snowball, n. & v.
--n.
1 snow pressed together into a ball, esp. for throwing in play.
2 anything that grows or increases rapidly like a snowball rolled on snow.
--v.
1 intr. & tr. throw or pelt with snowballs.
2 intr. increase rapidly.

Idiom

snowball-tree a guelder rose.

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THESAURUS

snowball

accrue, accumulate, accumulation, acervation, advance, agglomerate, agglomeration, aggregate, aggregation, amassment, appreciate, augment, avalanche, ball, balloon, bead, blizzard, bloat, boom, breed, broaden, build, bulk, chunk, congeries, conglobation, conglobulate, conglomerate, conglomeration, crescendo, crystal, cumulation, develop, dilate, distend, driven snow, enlarge, expand, extend, fill out, flake, flurry, gain, gain strength, gathering, get ahead, globe, glomeration, go up, gob, granular snow, greaten, grow, hunk, igloo, increase, intensify, lump, mantle of snow, mass, mogul, mount, multiply, mushroom, proliferate, puff up, rise, run up, shoot up, slosh, slush, snow, snow banner, snow bed, snow blanket, snow blast, snow fence, snow flurry, snow roller, snow slush, snow squall, snow wreath, snow-crystal, snowbank, snowbridge, snowcap, snowdrift, snowfall, snowfield, snowflake, snowland, snowman, snowscape, snowshed, snowslide, snowslip, snowstorm, sphere, spherify, spread, stockpile, strengthen, stretch, swell, tumefy, upsurge, wad, wax, wet snow, widen

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ROGET THESAURUS

snowball

Assemblage

N assemblage, collection, collocation, colligation, compilation, levy, gathering, ingathering, muster, attroupement, team, concourse, conflux, congregation, contesseration, convergence, meeting, levee, reunion, drawing room, at home, conversazione, assembly, congress, convention, conventicle, gemote, conclave, posse, posse comitatus, Noah's ark, miscellany, collectanea, museum, menagerie, museology, crowd, throng, group, flood, rush, deluge, rabble, mob, press, crush, cohue, horde, body, tribe, crew, gang, knot, squad, band, party, swarm, shoal, school, covey, flock, herd, drove, atajo, bunch, drive, force, mulada, remuda, roundup, array, bevy, galaxy, corps, company, troop, troupe, task force, army, regiment, host, populousness, clan, brotherhood, fraternity, sorority, association, volley, shower, storm, cloud, group, cluster, Pleiades, clump, pencil, set, batch, lot, pack, budget, assortment, bunch, parcel, packet, package, bundle, fascine, fasces, bale, seron, seroon, fagot, wisp, truss, tuft, shock, rick, fardel, stack, sheaf, haycock, fascicle, fascicule, fasciculus, gavel, hattock, stook, accumulation, congeries, heap, lump, pile, rouleau, tissue, mass, pyramid, bing, drift, snowball, snowdrift, acervation, cumulation, glomeration, agglomeration, conglobation, conglomeration, conglomerate, coacervate, coacervation, coagmentation, aggregation, concentration, congestion, omnium gaterum, spicilegium, black hole of Calcutta, quantity, collector, gatherer, whip, whipper in, assembled, closely packed, dense, serried, crowded to suffocation, teeming, swarming, populous, as thick as hops, all of a heap, fasciculated cumulative, the plot thickens, acervatim, tibi seris tibi metis.

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