Punctuation is important. The Oxford comma can eliminate a lot of ambiguity and a well-placed semicolon can help to break up what would otherwise be a run-on sentence. You may also recall that hyphens can completely change the meaning of a word. “Recreation” may refer to leisure activities, whereas “re-creation” may refer to creating something again. So, does a term like “heads-up” require a hyphen? It depends on what you mean and how you’re trying to say it.
Heads-Up as a Noun
A common idiomatic phrase in English is to “give someone a heads-up” about something. This means that you want to warn somebody about something that is going to happen. You want to give them advance notice, providing them with information that you think they may find useful (so they can prepare for it). In this case, “heads-up” is a noun and it takes a hyphen.
Let’s say that you are tasked with bringing food to a party. Your friend Jake might give you a heads-up that some of the attendees are vegetarian. That way, you can plan the spread accordingly to accommodate these types of dietary requests. Maybe someone will give you a heads-up that Joan just went through a nasty break-up, so you might not want to discuss relationship matters with her.
Heads Up as an Imperative Verb
Things change when you choose to use the term as an imperative verb. An imperative verb functions in an imperative sentence where the speaker is expressing a command. Go there. Stop that. Try this dip. Go, stop and try are imperative verbs in these sentences.
In some sense, you could say that “heads up” is simply a much shorter and faster way of saying, “May I have your attention? You should lift your heads up and be aware of your surroundings, as there may be a danger headed in your general direction.”
It’s important to note that even if you are addressing a single person (with a single head), the expression maintains the plural form “heads.”
Heads Up as an Adjective
This is somewhat counterintuitive (another word that feels like it needs a hyphen but Merriam-Webster says it doesn’t), at least for me, but if you want to use “heads up” as an adjective, it should be written as two separate words without a hyphen.
The most common place where you’ll find this kind of usage is with “heads up poker,” where there are only two players at the table. This could refer to a game where there are only two players from the start or it could refer to when all but two players have been eliminated from the poker table.
While deciding between child-care and childcare could be based on the difference between British and American English, choosing whether or not to hyphenate “heads up” has more to do with the part of speech. Just a heads-up in case this ever comes up in your writing.