Bone up on
To study hard, usually in preparation for a test.
There are two chief theories as to the origin of this phrase. One is that it derives from the practise of using bones to polish leather. So, to 'bone up' on a subject was to polish or refine one's knowledge. The second theory relates to the Victorian bookseller Henry George Bohn (1796-1884). He produced a large catalogue of books, including many study texts.
Early citations of the phrase in print, of which there are very few, don't support either idea. Bone was used as a verb meaning 'to study' from the early 19th century onward. The first known citation that explicitly use 'bone up' is in Tenting on Plains by Elizabeth Custer (wife of General George Custer), 1887:
"I have known the General to 'bone-up', as his West Point phrase expressed it, on the smallest details of some question at issue."
The Bohn story has the feel of something retro-fitted to the facts. If it really were true we might expect to find some 19th century reference that linked Bohn name with the phrase, or some example of 'Bohn up' in print. Nevertheless, the term must have come from somewhere, so the polishing with bone seems the most probable. Without further evidence the origin remains uncertain.