Old English hearm "hurt, pain; evil, grief; insult," from Proto-Germanic *harmaz (source also of Old Saxon harm, Old Norse harmr "grief, sorrow," Old Frisian herm "insult; pain," Old High German harm, German Harm "grief, sorrow, harm"), from PIE *kormo- "pain." To be in harm's way is from 1660s.
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.).
<a href="https://www.etymonline.com/word/harmful">Etymology of harmful by etymonline</a>
Harper, D. (n.d.). Etymology of harmful. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/harmful