n.1 in senses of WARN v.
2 anything that serves to warn; a hint or indication of difficulty, danger, etc.
3 archaic NOTICE n. 3b.
Phrases and idioms:
warning coloration Biol. conspicuous colouring that warns a predator etc. against attacking.
Derivatives:
warningly adv.
Etymology: OE war(e)nung etc. (as WARN, -ING(1))