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:
https://polishforums.com/polonia/satellite-abroad-44158/#msg909602When 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?