c. 1300, agheful, aueful, "worthy of respect or fear, striking with awe; causing dread," from aghe, an earlier form of awe (n.), + -ful. The Old English word was egefull. The weakened sense of "very bad" is by 1809; the weakened sense of "excessively; very great" is by 1818. It formerly also was occasionally used in a sense of "profoundly reverential, full of awe" (1590s).
also godawful, according to OED from 1878 as "impressive," 1897 as "impressively terrible," but it seems not to have been much in print before c. 1924, from God + awful. The God might be an intensifier or the whole might be from the frequent God's awful (vengeance, judgment, etc.), a common phrase in religious literature.
"causing or attended by great fear, dreadful, awful," 1560s, from Latin dirus "fearful, awful, boding ill," a religious term, which is of unknown origin. Apparently a dialect word in Latin; perhaps from Oscan and Umbrian and perhaps cognate with Greek deinos "terrible," Sanskrit dvis- 'hate, enmity, enemy," from PIE root *dwei-, forming words for "fear; hatred."
1630s, "awful, dreadful, terrible," from Latin tremendus "fearful, to be dreaded, terrible," literally "to be trembled at," gerundive form of tremere "to tremble" (see tremble (v.)). Hyperbolic or intensive sense of "extraordinarily great or good, immense" is attested from 1812, paralleling semantic changes in terrific, terrible, dreadful, awful, etc. Related: Tremendously.
Old English unhalig, "impious, profane, wicked," from un- (1) "not" + halig (see holy). Similar formation in Middle Dutch onheilich, Old Norse uheilagr, Danish unhellig, Swedish ohelig. In reference to actions, it is attested from late 14c. Colloquial sense of "awful, dreadful" is recorded from 1842.
also awestruck, "overwhelmed by reverential fear," 1630s (Milton), from awe (n.) + struck (see strike (v.)). Perhaps coined to cut a path between the contemporary senses of awesome ("reverential") and awful ("causing dread"). Awe-strike (v.) is not recorded until much later (1832), has always been rare, and is perhaps a back-formation.
c. 1300, "terrifying, horrible, dreadful," from Anglo-French hidous, Old French hideus, earlier hisdos "hideous, horrible, awful, frightening" (11c.; Modern French hideux), from hisda "horror, fear," perhaps of Germanic origin. The old guess that it comes from Vulgar Latin *hispidosus, from Latin hispidus "shaggy, bristly," "presents numerous difficulties" [OED] and seems now to be generally discredited. Meaning "repulsive" is late 14c.
early 12c., from Old English gemot "meeting, formal assembly" (especially of freemen, to discuss community affairs or mete justice), "society, assembly, council," from Proto-Germanic *ga-motan (compare Old Low Frankish muot "encounter," Middle Dutch moet, Middle High German muoz), from collective prefix *ga- + *motan (see meet (v.)). In early 15c. awful moot was used for "the Last Judgment."
late 14c., "causing terror, awe, or dread; frightful," from Old French terrible (12c.), from Latin terribilis "frightful," from terrere "fill with fear," from PIE root *tros- "to make afraid" (source also of Sanskrit trasanti "to tremble, be afraid," Avestan tarshta "scared, afraid," Greek treëin "to tremble, be afraid," Lithuanian trišėti "to tremble, shiver," Old Church Slavonic treso "I shake," Middle Irish tarrach "timid"). Weakened sense of "very bad, awful" is first attested 1590s.