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