Heads up is defined as something you shout to get someone's attention.
An example of heads up is when you yell at a person who isn't paying attention and who is about to get hit with a frisbee.
The definition of a heads up is an alert or a warning.
An example of a heads up is when you let your Internet-surfing co-worker know that your boss is about to enter his office.
heads-up definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
alert and resourceful: playing heads-up baseball
Origin: ; from interjectional phrase heads up!: see head
the act or an instance of providing a notice or warning to alert someone, as in advance of difficulty or trouble
heads-up definition by American Heritage Dictionary
Used as a warning to watch out for a potential source of danger, as at a construction site.
The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Showing an alert, competent style: play heads-up basketball. noun Informal
Information or notification, especially in advance: gave me the heads-up on the new security measures.
heads-up - Phrases/Idioms
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]