c. 1600, from event + -ful. According to OED, it is in Shakespeare, once ("As You Like It"), and there is no record of it between then and Johnson's "Dictionary." Related: Eventfully; eventfulness. Eventless is attested from 1815.
1570s, "the consequence of anything" (as in in the event that); 1580s, "that which happens;" from French event, from Latin eventus "occurrence, accident, event, fortune, fate, lot, issue," from past participle stem of evenire "to come out, happen, result," from assimilated form of ex- "out" (see ex-) + venire "to come" (from a suffixed form of PIE root *gwa- "to go, come"). Meaning "a contest or single proceeding in a public sport" is from 1865. Events as "the course of events" is attested from 1842. Event horizon in astrophysics is from 1969.
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.).