Englishoffhand
adjective
an offhand comment that she regretted almost immediately:
indifferent, casual, careless, uninterested, unconcerned, cool, distant, aloof, nonchalant, blasé, insouciant, offhanded, cavalier, glib, perfunctory, cursory, unceremonious, ungracious, curt, abrupt, terse, brusque, dismissive, discourteous, uncivil, impolite, rude;
impromptu, off-the-cuff, spontaneous, extempore, unpremeditated, extemporaneous, unthinking, unstudied;
informal off, offish, couldn't-care-less, take-it-or-leave-it;
rare pococurante.
adverb
I can't think of a better answer offhand:
without preparation, on the spur of the moment, without consideration, without checking, extempore, impromptu, ad lib;
extemporaneously, without rehearsal, spontaneously;
Latin ad libitum;
informal off the cuff, off the top of one's head, just like that, at the drop of a hat.
offhand Oxford Dictionary of English