Bunberry

verb. Inventing an excuse to avoid obligations (such as dinner parties with insufferable aunts). esp. an excuse that involves travel

etymology: in The Importance of Being Ernest by Oscar Wilde the character initially called "Algernon" invents a friend named "Bunberry" who is an invalid plagued by sporadic, inexplicable disease, and whom Algernon visits regularly.

Mr. Williams left town to visit his sick friend, perhaps he's Bunberrying and really seeing a mistress.