Harmful

wiktionary

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From Middle English harmful, from Old English *hearmful (suggested by hearmfullīċ(“harmful; hurtful”)), equivalent to harm +‎ -ful.

Cognate with German harmvoll, Danish harmfuld, Swedish harmfull.

etymonline

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harmful (adj.)

mid-14c., from harm (n.) + -ful. Related: Harmfully. Old English had hearmful but the modern word probably is a Middle English formation.