Other forms: startling, startled, startles
To startle somebody is to make them jump. A hand on the shoulder they never heard coming.
The word is about the body rather than the emotion: a startled person has flinched before they have had time to think. That is why it suits animals so well — a startled deer, a startled bird going up out of a hedge. The two adjectives divide the work neatly, since startled describes a face and startling describes news. Something startling is surprising enough to stop you, and it turns into a compliment when applied to talent or to beauty.
Also accepted in games: surprise
Startle is start with the ending that means to keep doing it. Old English steartlian meant to kick, to struggle or to stumble, so startling somebody makes them start over and over.
Old English
steartlian — to kick with the foot, struggle, stumble
Middle English
startlen
English, today
startle
Built from
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.
Etymology from Wiktionary, CC BY-SA