An AP story about clothing-free skater Gennifer Moss in Portland, Ore. -- who was told to put on some clothes after some construction workers complained about her -- contained an a phrase that I've always wondered about: "Moss donned a string bikini bottom for the nonce and skated on." The phrase means "for the occasion," according to the American Heritage Dictionary. (The OED gives a slightly different, but similar definition of "for the particular purpose.")
It comes from an alteration of the early Middle English phrase "for then anes," which appeared as early as 1175, according to the OED. The alteration changed it to "for the nanes," which became for the nonce. "For then anes" meant "for the once," according to the American Heritage Dictionary, which explains that "then" was a form of the at the time.
CORRECTION: An changed to a in first paragraph. Thanks to Dr. Trayes for pointing it out.