Startle
Old English steartlian ‘kick, struggle’, from the base of start. The early sense gave rise to ‘move quickly, caper’ (typically said of cattle), whence ‘cause to react with fear’ (late 16th century).
wiktionary
From Middle English startlen, stertlen, stertyllen(“to rush, stumble along”), from Old English steartlian(“to kick with the foot, struggle, stumble”), equivalent to start + -le. Cognate with Old Norse stirtla(“to hobble, stagger”), Icelandic stirtla(“to straighten up, erect”). Compare also Middle English stertil(“hasty”). More at start.
etymonline
startle (v.)
c. 1300, " move agitatedly, run to and fro" (intransitive), also "caper, romp, skip; leap, jump;" from Old English steartlian, from the source of start (v.) + frequentative suffix -le (as in topple, jostle, fizzle, etc. Sense of "move suddenly in surprise or fear" first recorded 1520s. Transitive meaning "frighten suddenly" is from 1590s. The word retains more of the original meaning of start (v.). Related: Startled; startling; startlingly. As a noun from 1714.
