The verb 'startle' originates from the frequentative form of the now-obsolete Middle English verb 'startlen', itself derived from Old English steartlian, meaning 'to kick, struggle, or stumble'. This Old English term is related to 'start' in its older sense of a sudden movement. The semantic journey moved from the physical act of stumbling or leaping to the more specific involuntary reaction caused by sudden fear or surprise. The modern abstract meaning retains the core concept of a sudden, reflexive physical or emotional jerk.

This is a simplex word of Germanic origin. The Old English root is steartlian, meaning 'to kick, struggle, or stumble'. The core functional role is as a verb describing a sudden, uncontrolled physical reaction.