What does it mean to be a Boswell?
To be someone's Boswell is a phrase inspired by the relationship between James Boswell and Samuel Johnson. Boswell looked up to the older and more famous Johnson, and he made a point of writing down many of their conversations and accompanying Johnson on some of his travels, especially their trips to the Hebrides in 1773. In 1791, Boswell published the biography The Life of Johnson about his friend, who had died 7 years earlier. The Life has had as much influence on later biographies, as Johnson's own dictionary has had on the history of dictionaries, and being someone's Boswell has come to mean a frequent companion and life chronicler of that person.
The most famous example of "being someone's Boswell" is probably in Arthur Conan Doyle's A Scandal in Bohemia (1891), in which Sherlock Holmes says to Dr. Watson, "I am lost without my Boswell." In Conan Doyle's novels about the famous detective, Watson assists Holmes on most of his investigations, and he is also the supposed author of the novels themselves, thus being Holmes' chronicler and companion.