From NYTimes.com:
In most of the Spanish-speaking world, the principal ways to say “you” are the casual “tú,” and the formal “usted.” But in Colombia there is another “you” — “su merced,” meaning, “your mercy,” “your grace” or even “your worship,” and now contracted to the more economical “sumercé.”
Back when I got my first exposure to written Spanish, somewhere in the hallowed classrooms of MacArthur Elementary School, the abbreviation for “Usted” was Vd., and Vds. meant “Ustedes”.
Usted was a contraction for Vuestra Merced. Sometime, probably between the fourth grade and high school, the popular contractions changed to Ud. and Uds.
When I pass from this mortal coil, all this trivial information may be lost forever.