What does the idiom he tried to bone up his vocabulary before test?

The idiom "bone up" is a slang term that means to study intensively or cram for something, usually an exam or test.

So, "he tried to bone up his vocabulary before the test" means that he tried to learn and memorize a lot of vocabulary words in a short amount of time before the test.

It's important to note that "bone up" is informal and can sound a bit crude. In formal settings, you might say "he tried to brush up on his vocabulary" or "he tried to review his vocabulary" instead.