Word Study
heads-up
CIDE DICTIONARY
heads-up, adj.
maintaining presence of mind; alert and attentive; able to recognize and take quick advantage of opportunities; resourceful; as, he played good heads-up baseball. [WordNet 1.5]
heads-up, n. [From its use as an interjection to warn of impending danger.].
a warning to be prepared for an imminent event. [PJC]
For further exploring for "heads-up" in Webster Dictionary Online