The Collaborative International Dictionary of English v.0.48:

Casual \Cas"u*al\, a. [OE. casuel, F. casuel, fr. L. casualis,
   fr. casus fall, accident, fr. cadere to fall. See Case.]
   1. Happening or coming to pass without design, and without
      being foreseen or expected; accidental; fortuitous; coming
      by chance.
      [1913 Webster]

            Casual breaks, in the general system. --W. Irving.
      [1913 Webster]

   2. Coming without regularity; occasional; incidental; as,
      casual expenses.
      [1913 Webster]

            A constant habit, rather than a casual gesture.
                                                  --Hawthorne.

   Syn: Accidental; fortutious; incidental; occasional;
        contingent; unforeseen. See Accidental.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Casual \Cas"u*al\, n.
   One who receives relief for a night in a parish to which he
   does not belong; a vagrant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

casual
    adj 1: marked by blithe unconcern; "an ability to interest
           casual students"; "showed a casual disregard for cold
           weather"; "an utterly insouciant financial policy"; "an
           elegantly insouciant manner"; "drove his car with
           nonchalant abandon"; "was polite in a teasing nonchalant
           manner" [syn: casual, insouciant, nonchalant]
    2: without or seeming to be without plan or method; offhand; "a
       casual remark"; "information collected by casual methods and
       in their spare time"
    3: appropriate for ordinary or routine occasions; "casual
       clothes"; "everyday clothes" [syn: casual, everyday,
       daily]
    4: occurring or appearing or singled out by chance; "seek help
       from casual passers-by"; "a casual meeting"; "a chance
       occurrence" [syn: casual, chance(a)]
    5: hasty and without attention to detail; not thorough; "a
       casual (or cursory) inspection failed to reveal the house's
       structural flaws"; "a passing glance"; "perfunctory courtesy"
       [syn: casual, cursory, passing(a), perfunctory]
    6: occurring from time to time; "casual employment"; "a casual
       correspondence with a former teacher"; "an occasional worker"
       [syn: casual, occasional]
    7: characterized by a feeling of irresponsibility; "a broken
       back is nothing to be casual about; it is no fooling matter"
       [syn: fooling, casual]
    8: natural and unstudied; "using their Christian names in a
       casual way"; "lectured in a free-and-easy style" [syn: free-
       and-easy, casual]
    9: not showing effort or strain; "a difficult feat performed
       with casual mastery"; "careless grace" [syn: casual,
       effortless]

Moby Thesaurus II by Grady Ward, 1.0:

355 Moby Thesaurus words for "casual":
   Bohemian, Charley, Mickey Mouse, VC, Vietcong, accessory,
   accidental, accidentally, additional, adscititious, adventitious,
   affable, aimless, airy, aleatory, almsman, almswoman, aloof,
   amorphous, apathetic, appurtenant, ascititious, ataractic,
   automatic, auxiliary, bankrupt, beggar, blanket, blase,
   blue-collar worker, breadwinner, breezy, bushfighter, bushwhacker,
   by-the-way, capricious, careless, casual laborer, casually,
   causeless, chance, chancy, charity case, circumstantial, clear,
   collateral, common, common laborer, conditional, contingent, cool,
   cordial, cursory, cushy, day laborer, degage, designless, destinal,
   desultory, detached, devil-may-care, dicey, disarticulated,
   discontinuous, disinterested, disjunct, disordered, dispassionate,
   dispersed, disproportionate, disregardant, disregardful,
   down-and-out, down-and-outer, driftless, dysteleological, easy,
   easy as pie, easygoing, effortless, employee, en deshabille,
   erratic, extemporaneous, extempore, extra, facile, factory worker,
   familiar, fatal, fatidic, fitful, flippant, fluky, flunky, folksy,
   forgetful, formless, fortuitous, fortuitously, free and easy,
   free lance, free-lancer, frivolous, full-time worker, general,
   glib, gracious, gratuitous, guerrilla, gut, hand, haphazard,
   happy-go-lucky, hardcase, haymish, heedless, hit-or-miss, homely,
   homey, iffy, ill-advised, ill-considered, ill-devised,
   immethodical, impromptu, improvised, imprudent, impulsive,
   in dishabille, inadvertent, inattentive, inchoate, incidental,
   incoherent, inconsiderable, inconsiderate, incurious, indeliberate,
   indeterminate, indifferent, indigent, indiscreet, indiscriminate,
   indiscriminative, industrial worker, inessential, inexcitable,
   inexplicable, informal, insensitive, insignificant, insouciant,
   involuntary, irregular, jobber, jobholder, laborer, laboring man,
   lackadaisical, lazy, light, listless, loose, lounge, low-pressure,
   maquis, maquisard, meaningless, menial, migrant, mindless, minor,
   misshapen, moiler, natural, navvy, negligent, nonchalant,
   nonessential, nonjudgmental, nonsystematic, nonuniform,
   nothing to it, oblivious, occasional, odd, off, offhand, offhanded,
   office temporary, orderless, out-of-the-way, painless,
   parenthetical, part-time, partisan, pauper, penniless man,
   perfunctory, petty, plain, planless, pococurante, poor devil,
   poor man, poorling, proletarian, promiscuous, provisional,
   purposeless, random, reckless, reflex, reflexive, regardless,
   relaxed, remote, resistance, resistance fighter, respectless,
   risky, roustabout, salaried worker, secondary,
   self-employed person, senseless, serendipitous, servant, shapeless,
   shoestring, side, simple, simple as ABC, small-beer, smooth, snap,
   sociable, soft, spare, spasmodic, spontaneous, sporadic, sporty,
   starveling, stiff, stochastic, straggling, straggly,
   straightforward, stray, subsidiary, superadded, superfluous,
   supervenient, supplemental, supplementary, systemless, tactless,
   temporary, thoughtless, toiler, trained, trivial, turned-off,
   unaccountable, unadvised, unaffected, unanxious, unarranged,
   unassuming, unburdensome, uncalculated, uncaused, unceremonious,
   uncertain, unclassified, uncomplicated, unconcerned, unconscious,
   unconsidered, unconstrained, unconventional, uncritical,
   uncriticizing, undeliberate, undeliberated, undemanding,
   underground, underground fighter, undesigned, undetermined,
   undifferentiating, undiplomatic, undirected, undiscreet,
   undiscriminating, undiscriminative, unessential, unexacting,
   unexpected, unfastidious, unforeseeable, unforeseen, unfussy,
   ungraded, unheedful, unheeding, unimportant, unintended,
   unintentional, uninterested, unjoined, unlooked-for, unmeditated,
   unmethodical, unmeticulous, unmindful, unmotivated, unofficial,
   unordered, unorganized, unparticular, unplanned, unpredictable,
   unpremeditated, unprepared, unready, unselective, unsolicitous,
   unsorted, unstudied, unsubtle, unsure, unsymmetrical, unsystematic,
   untactful, unthinking, ununiform, unwilled, vague, wage earner,
   wage slave, wageworker, wandering, welfare client, wholesale,
   withdrawn, worker, workgirl, workhand, working girl, workingman,
   workingwoman, workman



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CASUAL. What happens fortuitously what is accidental as, the casual
revenue's of the government, are those which are contingent or uncertain.