Warn  v. 1 (also absol.) A (often foll. By of or that) inform of danger, unknown circumstances, etc. B (foll. By to + infin.) Advise (a person) to take certain action. C (often foll. By against) inform (a person etc.) About a specific danger. 2 (usu. With neg.) Admonish.  warn off tell (a person) to keep away (from). [old english]