An accronym, stands for: So F*cking Dead To Me. To be used in regular online conversations when it gets too annoying to spell it all out. Usually occurs after someone pisses you off, forgets something for you, or was just born to annoy you.
'Friend': Hey, sorry I forgot you at the pub. I got pissed and then ended up going home with a stranger. Where were you?
Me: Sod off, you're SFDTM.