Heads-up or Head-ups?

I am such an OC when it comes to spellings and pronunciations (though grammar is not something that I would consider myself an expert of). I sent an e-mail earlier to an office mate and used the word "heads up." She replied using the word "head ups." This got me thinking, which one is correct?

                   heads up vs. head ups

Apparently, none of the above is correct. There should be a dash (-) in between.

TheFreeDictionary defines this as:

          heads-up (hdzp)

          adj.

      Showing an alert, competent style: play heads-up basketball.

          n. Informal

      Information or notification, especially in advance: gave me the heads-up on the new security measures.

So, there...I was able to review on my spelling today.