To notify others in advance of imminent dangers or risks: The flashing road signs were placed near the construction site to forewarn drivers of possible harm or injury and encouraging them to slow down and to avoid accidents.

Advanced information of an impending response: Walter had ample forewarning of the reception his report would receive.

Relating to someone who informs others in advance of an impending or possible danger, problem, or of other unpleasant situations: Tim's teacher spoke decisively in the most forewarning manner that cheating on the final exam would not be tolerated.