IPA: /ˈwɔːrnər/
KK: /ˈwɔrnər/
A person who gives a warning or alerts others about something that may happen.
The warner advised the community about the approaching storm.
Warner → It is formed from "warn" (from Old English *warnian*, meaning to give notice or to advise) and "-er" (meaning a person who). A warner is a person who gives notice or advice, often to alert others to danger.
Think of someone who 'gives notice' ('warn') about something important — that's why a warner is a person who alerts others.