It's a boat term, not internet shorthand
s/v stands for sailing vessel.... m/v stands for motor vessel...and MY stands for motoryacht. And while we're on the subject...The term "yacht" has many definitions. Technically, a "yacht" is any vessel--sail or power--used solely for pleasure. To a Brit, however, only sailing vessels used for pleasure are "yachts"...motor boats are just motorboats. In the US, the term "yacht" is usually reserved for BIG pleasure craft--power and sail...Chapman's says that a boat becomes a "yacht" at 40', but that hasn't stopped some boat mfrs from calling anything with a cabin a "motoryacht."All the etiquette books say that one must NEVER refer to one's own vessel as "my yacht." Even if it's 110' floating palace, one must refer it as "my (or our) BOAT" unless one wishes to be branded as gauche and socially unacceptable...at least in the US. However, as noted above, Brits view it differently.Gauche or not, my favorite definition of "yacht" is: how you describe your boat to people you know will never see it.