A warning or call to pay attention; an advisory notice; a notice of what is to happen; a holler.
Send everyone a heads-up about the inspection tomorrow.
Involving two players. (usually said when there are only two players in the table; or, when all players except two folded)
There are seven players on the table. Five folded, the remaining two played heads-up until the showdown.
With head directly above the neck, eyes front.
A heads-up posture meant the pilot couldn't pay attention to his instruments.
Alert; vigilant
Getting those marbles off the red carpet was a real heads-up move.