"characterized by inattention or indifference," 1640s, from neglect (n.) + -ful. Related: Neglectfully; neglectfulness. Earlier in same sense was neglective (1610s).
Entries linking to neglectful
neglect n.
1580s, "act of treating with slight attention;" 1590s, "omission, oversight, want of attention to what ought to be done;" from neglect (v.) or from Latin neglectus "a neglecting," noun use of past participle of neglegere.
-ful
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.).