Give a head(s) up grammar

[duiter]

Hi all,

Why '' Give a heads up " idiom uses pural form of heads after following "a" ?

Why not "Give a head up" ?

I know the meaning, I just want to know the grammar of this idiom

Many thanks

[Verona_82]

Not a teacher.
"Heads-up" is a singular noun. The indefinite article refers to the noun as a whole: "a heads-up".
I guess the phrase 'heads up!" was originally used to warn people about something or make them pay attention to something - indeed, one should hold their head up so as to be able to see what's going on.
Here's some information about the origin: Heads up

Now I'm wondering if the plural form is ever used and which is it - 'heads-up" or "heads-ups"?