gruesome (adj.)

word-forming element used in making adjectives from nouns or adjectives (and sometimes verbs) and meaning "tending to; causing; to a considerable degree," from Old English -sum, identical with some, from PIE root *sem- (1) "one; as one, together with." Cognate with Old Frisian -sum, German -sam, Old Norse -samr; also related to same. "It usually indicates the possession of a considerable degree of the quality named: as mettlesome, full of mettle or spirit; gladsome, very glad or joyous" [Century Dictionary]. For the -some used with numbers (twosome, foursome, etc.), see -some (2).

Old English grislic (in compounds) "horrible, dreadful," from root of grisan "to shudder, fear," a general Germanic word (cognates: Old Frisian grislik "horrible," Middle Dutch grisen "to shudder," Dutch griezelen, German grausen "to shudder, fear," Old High German grisenlik "horrible;" of unknown origin; Watkins connects it with the PIE root *ghrei- "to rub," on notion of "to grate on the mind." See also gruesome, to which it probably is connected in some way. Related: Grisliness.