He quit his job on the spur of the moment without having another one lined up.

on the spur of the moment 是一个固定短语，意思是"一时冲动、未经深思熟虑地做某事"。 spur 的本义是"马刺"（骑马时用来刺激马匹的金属配件），引申为"刺激、激励"。整个短语借用"马刺一刺、立刻催马出发"的形象，比喻人在某种突然的冲动下立即行动、不作计划。