on the spur the moment (adverb) extemporaneously: without preparation
Sinonime: adventitious, casual, chance,
coincidental,
contingent, fluky, fortuitous, inadvertent,
incidental, random, uncalculated, unexpected, unforeseen, unintended,
unintentional, unplanned
Antonime: decided, designed,
essential, intended, intentional, planned, premeditated