What is the origin of the idiom "heads-up" and when to use it?

The origin of the idiom "heads-up" is rooted in early 20th-century military and aviation terminology. It was initially used by military personnel to warn about incoming threats, such as enemy aircraft or artillery shells. The phrase likely originated from the idea that soldiers and aviators needed to keep their heads up and be alert to potential dangers from above. This expression is commonly used to provide someone with advance notice, warning, or information about a situation or development that requires their attention or action. It is often used to alert someone to be vigilant, prepared, or cautious regarding something that may affect them or their plans.