[hinkley]

But it's not a thing in movies, so nobody wonders why they say that in movies.

Because nobody says that in movies.

It's been 'synchronize watches' since WWII movies became a thing.

[ksaj]

Scroll through the comments. Someone quoted a MASH scene that uses it. It's a TV show and not a movie, but at least it's an example in the wild.

[hinkley]

MASH is culturally important but it’s not The Guns of Navarone or A Bridge Too Far. If Bridge on the River Kwai had watches, Sir Alec Guinness would have not yelled hack.

[wrp]

I've seen practically every B-grade and up WWII movie made in English, and I don't remember ever seeing the use of "hack". Anybody please point to movie references if they know any.

[hinkley]

My memory isn’t what it used to be but I think this is the first time I’ve heard it too, if that wasn’t already clear from my tone above.

[rconti]

I've never heard "hack" in this context either. "On my mark!" is what I think of.