adj. 25B6; adjective an offhand manner : CASUAL, careless, uninterested, unconcerned, indifferent, cool, nonchalant, blasé, insouciant, cavalier, glib, perfunctory, cursory, unceremonious, ungracious, dismissive, discourteous, uncivil, impolite, terse, abrupt, curt; informal off, couldn't-care-less, take-it-or-leave-it.
25B6; adverb I can't think of a better answer offhand : ON THE SPUR OF THE MOMENT, without consideration, extempore, impromptu, ad lib; extemporaneously, spontaneously; informal off the cuff, off the top of one's head, just like that.