fit as a butcher's dog

Etymology
Based on the assumption that a butcher's dog is likely to be well fed because of the ready supply of its master's meat.

Adjective

 * 1)  Very fit; in good shape.
 * 2) * 1991 July 4, Mike Selvey, The Guardian, "Cricket: England out to make it seventh time lucky":
 * This time not only is Gooch as fit as a butcher's dog but England, through their efforts at Headingley and Lord's, are mentally attuned to the process of both winning and, crucially in the second match, not losing.