Forewarn - Vine's Expository Dictionary of New Testament Words
Forewarn[ 1,,G4302, prolego ]
with verbal forms as mentioned above, is translated I forewarn" and "I did forewarn," in the RV of Galatians 5:21, AV, "I tell (you) before" and "I have told (you) in time past;" here, however, as in 2 Corinthians 13:2; 1 Thessalonians 3:4 (See below), the RV marg., "plainly" is to be preferred to "beforehand" or "before" (See under FORETELL); the meaning in Galatians 5:21 is not so much that Paul prophesied the result of the practice of the evils mentioned, but that he had told them before of the consequence and was now repeating his warning, as leaving no possible room for doubt or misunderstanding; in 1 Thessalonians 3:4, the subject told before was the affliction consequent upon the preaching of the Gospel; in 1 Thessalonians 4:6, "we forewarned," the warning was as to the consequences of whatsoever violates chastity.
Note: In Luke 12:5 the verb hupodeiknumi, "to shew, teach, make known," is translated "will warn" in the RV (AV, "forewarn"). See EXAMPLE (B, NO. 2), SHEW, WARN.