from Wiktionary, Creative Commons Attribution/Share-Alike License.
superlativeform of gruesome: most gruesome.
Sorry, no etymologies found.
Young as he was he had already more than once looked on the face of death, and had felt no awe; he had listened to the gruesomest of tales, told not altogether without art, and had never moved a hair
He unfortunately landed only about four feet from her and got the gruesomest scare of her life.