I'm actually stumped as to what the other expression is.
It might be dated.... I've mostly (only?) heard it on old British comedies from previous decades, but if you put it into google with 'meaning' it shows both the 'buckle up' meaning and the idiomatic meaning....
What I'm thinking of is a bit dated but I wouldn't say the idiomatic meaning applies to Zapnij pas. This is why I'm confused. It might not be what you're thinking though and maybe someone else will get it.....
I wouldn't say the idiomatic meaning applies to Zapnij pas.
Without looking too hard on youtube I found a public service announcement (a couple actually) from British TV using the 'zapnij pas' meaning.... and at least one video discusses the other meaning as a phrasal verb (and I found an example from a very, very old british comedy, the Larkins, as well).
As this idiom is for non-Brits, I'll stay quiet for now and I'll comment on previous answers later. Still not sure if what I'm thinking that you're thinking is right or not....
What about a really hard reverse one? This is a regionalism in the US that i heard the first time a year or so ago (from a different part of the country where I've never been) but I liked it and have used it incessantly ever since.... Super hard to translate into Polish so I'm going with two... one more literal (ish) one more meaning (ish) but which is which?
The British expression 'belt up' can mean either 'zapnij pas(y)' or 'shut up!' 'zamknij siÄ™!'....
This is what I thought you meant. It's not used very often these days but I found a good example of its idiomatic meaning on PF by a poster called 'Stu' when I looked the other day:
When you asked the original question, " What idiomatic meaning does zapnij pas! have in Britain?" I presumed it meant fasten your seatbelt and not belt up as such.
In the UK the idiomatic meaning of 'fasten your seatbelt' would be 'prepare for something unexpected, a bumpy ride'. Quite often that would mean prepare for trouble, so in that sense I think both Lenka and Iron were right in their explanations.
Maybe I have got confused with translating 'zapnij pas' to fasten your seatbelt instead of 'belt up'? Or do both apply?
koń za sztukę
A horse apiece?
Corresponding to the US expression 'It's a horse a piece' which means six of one, half a dozen of the other. It's doesn't matter much either way, something is more or less equal.
po koniu na każdego
A horse for everyone? Is this the more literal meaning?
I didn't see another way but I think I made the mistake of only looking at 'fasten your seatbelt' whereas 'belt up' also applies. Two idioms for the price of one!
It's hard to tell because it's not clear if each gets a horse or each alternative requires a horse...
For me it reads like each gets a horse.
I must say it's nice to see other people taking an interest and participating in this thread :)