I think it refers to grinding grains, which is a long time consuming job. Women used to spend a large portion of their day grinding the wheat to make the family bread. it is dull repetitive work.
Grinding with object Reduce (something) to small particles or powder by crushing it.
‘grind some black pepper over the salad’
‘she ground up the rice prior to boiling’
1.1 Sharpen, smooth, or produce (something) by crushing or by friction. ‘power from a waterwheel was used to grind cutlery’
1.2 Operate (a mill or machine) by turning the handle. ‘she was grinding a coffee mill’
1.3 (of a mill or machine) work with a crushing action. ‘the old mill was grinding again’
And source for how much time was taken:
Respect for our ancestors! To feed a family, someone had to do this
for a couple of hours—every day. Even taking into account that they
would have more experience, that was still a lot of work and very
straining on your body.