The Collaborative International Dictionary of English v.0.48:

Nonchalant \Non`cha`lant"\, a. [F., fr. non not (L. non) +
   chaloir to concern one's self for, fr. L. calere to be warm,
   to be inflamed with desire, to be troubled. See Non-, and
   Caldron.]
   Indifferent; careless; cool.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonchalant
    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]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "nonchalant":
   Laodicean, Olympian, aloof, apathetic, at ease, ataractic,
   benumbed, blah, blase, calm, careless, casual, cheerful, collected,
   comatose, composed, cool, degage, desensitized, detached,
   devil-may-care, disinterested, dispassionate, disregardful,
   distant, dull, easy, easygoing, effortless, free and easy, glad,
   happy-go-lucky, heartless, hebetudinous, heedless, hopeless,
   imperturbable, in a stupor, inattentive, incurious, indifferent,
   inexcitable, insouciant, lackadaisical, languid, lethargic, light,
   lighthearted, listless, mindless, negligent, numb, numbed, offhand,
   passive, perfunctory, phlegmatic, pluckless, pococurante, reckless,
   regardless, relaxed, resigned, slack, sluggish, smooth, soporific,
   spiritless, spunkless, stoic, stupefied, supine, together, torpid,
   turned-off, unanxious, uncaring, unconcerned, undiscriminating,
   undisturbed, unemotional, unenthusiastic, unexcitable, unexcited,
   unflappable, uninterested, unmindful, unperturbed, unruffled,
   unsolicitous, untroubled, withdrawn