The Collaborative International Dictionary of English v.0.48:

Offhand \Off"hand`\, a.
   Instant; unprepared; ready; extemporaneous; unrehearsed; as,
   an offhand speech; offhand excuses; an offhand comment. --
   adv. In an offhand manner; as, he replied offhand.
   [1913 Webster]

WordNet (r) 3.0 (2006):

offhand
    adv 1: without previous thought or preparation; "couldn't give
           the figures offhand"; "we decided offhand to go to
           Canada"; "she had made these remarks offhandedly" [syn:
           offhand, offhanded, offhandedly]
    2: in a casually inconsiderate manner; "replied offhand, his
       mind a million miles away"; "she threw him over offhandedly
       without even a Dear-John letter" [syn: offhand,
       offhanded, offhandedly]
    adj 1: with little or no preparation or forethought; "his ad-lib
           comments showed poor judgment"; "an extemporaneous piano
           recital"; "an extemporary lecture"; "an extempore skit";
           "an impromptu speech"; "offhand excuses"; "trying to
           sound offhanded and reassuring"; "an off-the-cuff toast";
           "a few unrehearsed comments" [syn: ad-lib,
           extemporaneous, extemporary, extempore,
           impromptu, offhand, offhanded, off-the-cuff,
           unrehearsed]
    2: casually thoughtless or inconsiderate; "an offhand manner";
       "she treated most men with offhand contempt" [syn: offhand,
       offhanded]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "offhand":
   Bohemian, abrupt, ad lib, ad libitum, ad-lib, affable, airily,
   airy, aloof, any old way, any which way, anyhow, at short notice,
   at sight, blase, brusque, bunglingly, by ear, by the by,
   by the way, careless, carelessly, casual, casually, cavalier,
   clumsily, cool, cordial, cursorily, cursory, curt, degage,
   devil-may-care, disregardant, disregardful, disregardfully,
   distant, easy, easygoing, en famille, en passant, extemporaneous,
   extemporaneously, extemporarily, extemporary, extempore, familiar,
   familiarly, flippant, folksy, forgetful, forgetfully,
   free and easy, glib, gracious, haphazardly, haymish, heedless,
   heedlessly, helter-skelter, hit and miss, hit or miss, homely,
   homey, impromptu, improvisatorial, improvise, improvised,
   improviso, in passing, inattentively, incidentally, inconsiderate,
   inconsiderately, indifferent, informal, informally, insouciant,
   irregular, jury-rigged, lackadaisical, lazy, light-hearted, loose,
   makeshift, messily, natural, naturally, nonchalant, oblivious,
   off the cuff, off the hip, off-the-cuff, offhanded, offhandedly,
   on sight, once over lightly, out of hand, parenthetically,
   perfunctorily, perfunctory, plain, plainly, promiscuously,
   reckless, recklessly, regardless, regardlessly, relaxed, relaxedly,
   respectless, sans ceremonie, simple, simply, slapdash, sloppily,
   smooth, sociable, spur-of-the-moment, stopgap, superficial,
   superficially, tactless, tactlessly, thoughtless, thoughtlessly,
   unaffected, unaffectedly, unassuming, unassumingly, unceremonious,
   unceremoniously, unconcerned, unconstrained, unconstrainedly,
   unconventional, undiplomatic, ungracious, unguardedly, unheedful,
   unheedfully, unheeding, unheedingly, uninterested, unmindful,
   unmindfully, unofficial, unofficially, unpremeditated, unprepared,
   unready, unrehearsed, unsolicitous, unsolicitously, unstudied,
   untactful, unthinking, unthinkingly, unvigilantly, unwarily,
   without ceremony