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heads-up [or] heads up

parts of speech:
noun, adjective, interjection
part of speech: noun
definition: (informal) an advance warning that something will happen.
He gave me a heads-up that the supervisor was coming, which gave me time to get ready.
part of speech: adjective
definition: displaying quick thinking or resourcefulness; alert; resourceful.
A final heads-up play by the quarterback won this game.
part of speech: interjection
definition: used as a warning signal, esp. when something is moving towards a person's head or body.
Here, Joe! Catch! Heads up, Dan!