The term 'GUN A MOTOR' is a phrase used to describe the process of revving UP an engine. The two crossword solutions for this term are REV (3 letters) and REVUP (5 letters). REV is short for the verb 'REVOLVE', which means to rotate or spin around a central point. REVUP is a combination of the words 'REV' and 'UP', which together mean to increase the speed of rotation. Therefore, the term 'GUN A MOTOR' can be interpreted as increasing the speed of an engine's rotation in order to start it.