"notice beforehand of the consequences that will probably follow continuance in some particular course" [Century Dictionary], Old English warnung, verbal noun from warnian (see warn (v.)).
Entries linking to warning
warn v.
Old English warnian "to give notice of impending danger," also intransitive, "to take heed," from Proto-Germanic *warōnan (source also of Old Norse varna "to admonish," Old High German warnon "to take heed," German warnen "to warn"), from PIE root *wer- (4) "to cover." Related: Warned; warning.