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