Grind to a halt
Lose momentum and stop.
My punishment for watching daytime TV is that the BBC often broadcasts 'infotainment' type history shows that, as incidental asides from the main theme, explain the origin of some phrase or another. These usually turn out to be as reliable as those from the average tour guide, i.e. almost inevitably made-up stories that serve someone's purpose. A recent (December 2009) BBC programme, The Victorian Farm, included and interview with a miller who explained that, when the wind drops, the windmill's grindstones slow down and eventually stop turning - "hence the expression, grind to a halt". That seemed plausible and worth checking, although my enthusiasm was blunted rather by hearing that the miller could tell how well the flour was ground by rubbing it between finger and thumb - "hence the expression, rule of thumb" - which I knew to be incorrect. Anyway, I persevered with a little research and here's what I found...
Frustratingly, the origins of 'grind to a halt', or 'ground to a halt', are unclear. What is known is that the phrases aren't, as they might sound, mediaeval, but are of quite recent coinage. The earliest examples that I can find of either term in print is from The Nevada State Journal, December 1934, in an article titled 'Gridiron Season Grinds to Close In America Today':
Football will grind to a halt tomorrow in scattered sections of the country.
The lateness of the emergence of the phrase in print does tend to rule out windmilling as the source - the heyday of such being long past by the 1930s. In the journalist's choice of language there seems to have been an association of 'gridiron' and 'grind'. This is paralleled by a similar link between 'grind to a halt' and 'gridlock'.
As to an actual origin; I doubt that one will be found. 'Grind to a halt' is probably just a colloquial phrase that refers to something slowing down and stopping - what that thing was when the first person used the expression isn't really important, and is almost certainly lost to us. My bet is that it wasn't a windmill.