The definition of gruesome is something unpleasant or causing horror.(adjective)
An example of something that would be described as gruesome is a violent murder scene.
See gruesome in Webster's New World College Dictionary
Origin: < dial. grue, to shudder (< ME gruwen, akin to MHG < IE base *ĝhreu-, to grind down > grit) + -some
See gruesome in American Heritage Dictionary 4
Origin: Obsolete grue, to shudder (from Middle English gruen, from Middle Dutch grūwen or Middle Low German gruwen)
Origin: + -some1.
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