From Middle English *forewarnien, from Old English forewarnian (“to take warning beforehand; forewarn”), equivalent to fore- + warn. Cognate with German vorwarnen (“to warn, forewarn”), Swedish förvarna (“to forewarn”).
Some discourage this use, finding the term redundant, as a warning is necessarily in advance. However, considering the word's continued presence in the English language ever since the time of the Anglo-Saxons (when it was first coined), the legitimacy of such complaints is somewhat questionable.
Additionally, many others argue that forewarn is simple emphasis (rather than redundancy), has connotations of “well in advance” (“Watch out!” and “Watch your head!” are warnings, but not forewarnings), and has connotations of “correct prediction”, as in foretell. Both forewarn and warn are well-established words, with forewarn being attested since 1330.
- “Forewarning signs”, The Grammarphobia Blog, May 8, 2007