Startler

Part of speech: noun

Definitions

1. An individual or object that elicits a startling reaction or unexpected fear
2. A factor that provokes a significant surprise or jolt of fear in someone
3. An entity or thing that causes an unexpected shock or sudden fright

Etymology: "Startler" is a plain formation: the verb "startle" with the agent suffix "-er" attached, so it denotes someone or something that startles. The verb "startle" comes from Old English "steartlian," meaning to stumble or to move suddenly, and it shifted over the centuries toward the sense of a sudden fright. The noun built from it inherits that sense of abrupt, unexpected shock. The suffix does the work here. English has long used "-er" to turn verbs into nouns for doers or causes of an action, so a "teacher" teaches and a "startler" startles. Because the formation is so transparent, the word needs little explanation beyond its parts.