To work up for an examination; to grind up the subjects set,
and to grind into the memory the necessary cram. The allusion is to a
mill, and the analogy evident.
To grind one down.
To reduce the price asked; to lower wages. A knife, etc., is
gradually reduced by grinding.
To take a grind
is to take a constitutional walk; to cram into the smallest space
the greatest amount of physical exercise. This is the physical grind.
The literary grind is a turn at hard study.
To take a grinder
is to insult another by applying the left thumb to the nose and
revolving the right hand round it, as if working a hand-organ or
coffeemill. This insulting retort is given when someone has tried to
practise on your credulity, or to impose upon your good faith.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Grind from Infoplease: