Forewarn
Origin
From Middle English *forewarnien, from Old English forewarnian ("to take warning beforehand; forewarn"), equivalent to - + warn. Cognate with German vorwarnen ("to warn, forewarn"), Swedish förvarna ("to forewarn").
Full definition of forewarn
Verb
- To warn in advance.
- 1913, w, Lord Stranleigh Abroad Chapter 4, “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. …â€
Usage notes
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.
The dictionary of disagreeable English, Robert Hartwell Fiske, 2006, p. 160
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 predictionâ€, as in foretell. Both forewarn and warn are well-established words, with forewarn being attested since 1330.