Harmful means causing damage to health or to things. Harmful chemicals, harmful effects, potentially harmful.
It is the standard word in health and safety labelling, where harmful is a defined hazard category weaker than toxic. In ordinary use it covers anything that does damage — harmful gossip, harmful advice. Note harmless, which is its opposite and is used far more often, particularly in harmless fun; and that the compound is Old English, built exactly as it still is.
1adjectiveharmfulCausing damage to health or to things.
“Too much sun can be harmful to your skin.”
synonyms: damaging, injurious
harmful is a quality of causing damage.
Also accepted in games: damaging
Old English hearmful is harm plus -ful. German harmvoll and Danish harmfuld are built the same way, though the Germanic harm originally carried a sense of grief as well as injury.
Old English
*hearmful
Middle English
harmful
English, today
harmful
Built from
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.
Etymology from Wiktionary, CC BY-SA