1
general::
Phrase(s): Heads up!
Raise your head and look around you carefully for information or something that you need to see or avoid. • Heads up! Watch out for that door! • Heads up! There is a car coming.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
A warning to watch out for potential danger, as in Heads up, that tree is coming down now! The expression is generally in the form of an interjection. [c. 1940]
American Heritage Idioms