Abbreviation of the term "main stream".
Generally used to put someone down.
This put down could be directed at someone who listens to the top 40
or watches shows on a popular channel,
3 years after they originally aired on a lesser known, but better channel.
It could also be a general statement for someone who just does what is
"in", "hip" or "cool", or just because everyone else is doing it.
The opposite of what some fags would call "underground".
"Pussycat Dolls? That is so MS"
"You watch Dexter on channel 10?
Well I watched it on showcase, before it was cool.
Man, you are just so unbelievably MS!"