The Collaborative International Dictionary of English v.0.48:

heads-up \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.

   Syn: wide-awake.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

heads-up \heads-up\ n. [From its use as an interjection to warn
   of impending danger.]
   a warning to be prepared for an imminent event.
   [PJC]

WordNet (r) 3.0 (2006):

heads-up
    adj 1: fully alert and watchful; "played heads-up ball" [syn:
           heads-up, wide-awake]
    n 1: a warning message; "he sent a heads-up to the District
         Attorney"