fearful adj.

Middle English fere, from Old English fær "calamity, sudden danger, peril, sudden attack," from Proto-Germanic *feraz "danger" (source also of Old Saxon far "ambush," Old Norse far "harm, distress, deception," Dutch gevaar, German Gefahr "danger"), from PIE *pēr-, a lengthened form of the verbal root *per- (3) "to try, risk."

The sense of "state of being afraid, uneasiness caused by possible danger" developed by late 12c. Some Old English words for "fear" as we now use it were fyrhto, fyrhto; as a verb, ondrædan. Meaning "feeling of dread and reverence for God" is from c. 1400. To put the fear of God (into someone) "intimidate, cause to cower" is by 1888, from the common religious phrase; the extended use was often at first in colonial contexts:

Thus then we seek to put "the fear of God" into the natives at the point of the bayonet, and excuse ourselves for the bloody work on the plea of the benefits which we intend to confer afterwards. [Felix Adler, "The Religion of Duty," 1905]

word-forming element attached to nouns (and in modern English to verb stems) and meaning "full of, having, characterized by," also "amount or volume contained" (handful, bellyful); from Old English -full, -ful, which is full (adj.) become a suffix by being coalesced with a preceding noun, but originally a separate word. Cognate with German -voll, Old Norse -fullr, Danish -fuld. Most English -ful adjectives at one time or another had both passive ("full of x") and active ("causing x; full of occasion for x") senses.

It is rare in Old English and Middle English, where full was much more commonly attached at the head of a word (for example Old English fulbrecan "to violate," fulslean "to kill outright," fulripod "mature;" Middle English had ful-comen "attain (a state), realize (a truth)," ful-lasting "durability," ful-thriven "complete, perfect," etc.).