In British English it is a slang term for a Scottish person (it can also be used an adjective). It is derived from "Jock", a Scottish variant of the forename "Jack", and is now considered offensive or contemptuous by many people.
In British English, it is a dialect characterized by broad accent.
In American English it is a slang term for an athlete, typically one thought of as stupid. This term is especially common in secondary schools and college. The stereotypical Jock is obsessed with sports. It may have come from 'jock strap', slang for 'athletic supporter'.
In American English it is also a slang term for the penis. This meaning may be related to the previous definition, because an athletic Jock is considered to have a higher level of testosterone, the male hormone.
