1
general::
Impulsively, without prior preparation, as in He decided to join a tour to England on the spur of the moment. This expression alludes to the goading action of a spur to a horse. [Late 1700s]
American Heritage Idioms
2
general::
Phrase(s): on the spur of the moment
Fig. suddenly; spontaneously. • We decided to go on the spur of the moment. • I had to leave town on the spur of the moment.
McGrawhill's American Idioms And Phrasal Verbs