forewarn

Early 14th century, from Middle English *forewarnen (suggested by Middle English forewarned, forewarning, forewarner, etc.), from Old English forewarnian (“to take warning beforehand; forewarn”), from Proto-Germanic *furawarnōną (“to forewarn”), equivalent to fore- +‎ warn. Cognate with German vorwarnen (“to forewarn”), Swedish förvarna (“to forewarn”).

forewarn (third-person singular simple present forewarns, present participle forewarning, simple past and past participle forewarned)

1. To warn in advance.
   • 1576, George Whetstone, “The Ortchard of Repentance: ”, in The Rocke of Regard, , London: for Robert Waley, →OCLC; republished in J[ohn] P[ayne] Collier, editor, The Rocke of Regard,  (Illustrations of Early English Poetry; vol. 2, no. 2), London: Privately printed, , →OCLC, page 291:

     And ſure, although it was invented to eaſe his mynde of griefe, there be a number of caveats therein to forewarne other young gentlemen to foreſtand with good government their folowing yl fortunes; […]

   • 1913, Robert Barr, chapter 4, in Lord Stranleigh Abroad, →OL:

     “I came down like a wolf on the fold, didn’t I ?  Why didn’t I telephone ?  Strategy, my dear boy, strategy. This is a surprise attack, and I’d no wish that the garrison, forewarned, should escape. […]”

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.

• forewarned is forearmed

• “Forewarning signs”, The Grammarphobia Blog, May 8, 2007