Wiktionary
heavy legs
n. An alleged medical condition, described as an unpleasant sensation of pain and heaviness in the lower limbs.
Wikipedia
Heavy legs
Heavy legs is an alleged medical condition, described as an "unpleasant sensation of pain and heaviness" in the lower limbs. It is of French origin but is occasionally seen elsewhere.
Due to the limited recognition it has received, and because of its proliferation in the media and by the alternative health community, many medical practitioners believe it is a sign of hypochondriasis.
The cause may be venous insufficiency or peripheral arterial disease.
It was described by BBC reporter Emma Jane Kirby as "a curiously French complaint ... the illness you can only get in health-conscious France."