It is one of several early morning expressions employed in Britain. Others include, 'The crack of bird fart', or Sparrow's fart, or 'Up with the larks and down with the mucking out.' Note in Britain we'd typically say the 'arse' crack of dawn. As a marathoner, the phrase perfectly captures for me the sensibilty of early morning distance training runs.
It is one of several early morning expressions employed in Britain. Others include, 'The crack of bird fart', or Sparrow's fart, or 'Up with the larks and down with the mucking out.' Note in Britain we'd typically say the 'arse' crack of dawn. As a marathoner, the phrase perfectly captures for me the sensibilty of early morning distance training runs.