A busman's holiday is one in which a person spends his or her vacation time doing the same activities as he or she does for a living. A physician, for example, may take a busman's holiday to work in a free clinic in a Third World country.
Taking a busman's holiday may be viewed by others as taking no vacation at all, but for some people the thought of working voluntarily can indeed be a welcome change in their routines. This type of holiday often allows professionals to experience life from the perspective of their clients or customers. Instead of driving a bus for a living, a vacationing bus driver can ride the bus as a regular passenger and enjoy the scenery.
There are a few different theories on the origin of the phrase "busman's holiday". The version most commonly accepted hearkens back to the horse-driven omnibuses of 19th Century England. The drivers of these omnibuses often grew very attached to their particular team of horses pulling their carriage through town. During their days off, many of these omnibus drivers would disguise themselves as regular passengers in order to keep a critical eye on the relief drivers and the horses. It is said that the phrase "busman's holiday" arose from this practice of bus drivers spending their downtime riding the buses.
Another theory also has a few supporters in the etymology world. The term "busman's holiday" could have nothing to do with professional bus drivers after all. During the same time period as the horse-drawn omnibuses, teams of pickpockets worked the streets of London. One team member would distract the victim by engaging in conversation, while his or her partner picked the victim's wallet. The slang term for the chattier of the two pickpockets was buzzman or buzman. It is thought by some etymologists that a "buzman's holiday" would be no holiday at all, since criminals rarely take a break from criminal activity. Eventually the term would be changed to the more acceptable "busman's holiday," complete with a plausible alternative origin.