very heavy work

ver·y hea·vy work

(ver'ē hev'ē wŏrk)
A physical demand level described as the exertion of up to 100+ pounds of force occasionally, or up to 50+ pounds of force frequently, or 20+ pounds of force constantly to move objects.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
References in classic literature ?
That's easy enough, for Matthew doesn't go about looking for excitement by any means and never did, but he's not to do any very heavy work either and you might as well tell Matthew not to breathe as not to work.