on the spur of the moment (adverb) extemporaneously: without preparation
Sinonime: accidental, aimless, casual, chance, erratic, extemporaneous, fortuitous, haphazard, inadvertent, involuntary, purposeless, random, unconscious, undesigned, undevised, unexpected, unforeseen, unintended, unplanned, unpremeditated, unthinking, unthought, unwitting
Antonime: definite,
deliberate,
intended,
intentional,
planned