Hash Out Agreement
I don`t think the switch to out is necessarily due to «Thrash it out» (which means the Americans just dropped the t&a?). We have similar constructions like «walk it off», «talk it over», «work it out», etc. Instead of derogating, it may be that «hash out» is an expression that has developed and become popular, while «Thrash it out» has never imposed itself – or at least then expanded (conversely in British English). In the United States, hash-out means having a discussion, especially one that must lead to an agreement or solution. The corresponding phrase outside of North America is Thrash Out, which is used almost exactly the same way. Hash out is the preferred term in Canada, but Thrash Out is more common in Canadian publications than in the United States, where it hardly exists. Later, Hash Over appears and although I can`t find a specific date, it seems to appear before the hash out. The meaning is a bit like Rehash. The original meaning of the word hash was «chopping, cutting into small pieces» of the French chopper, from the old French axe means axe. It often had culinary benefits, a hashish was a crushed mixture of things, especially the flat browns hash, which was made from crushed potatoes….