Acronym for "sucks, doesn't it?"
Monday-
You: What'd you get on your test?
Me: I failed :(
You: Hahaha. Good job, fuckhead.

Friday-
You: Dude, I totally just failed my test in bio :(
Me: SDI. XD
by Margo Roth Spiegelman II April 15, 2009
imaginary instatute made up by a 1337 master of "tai qwan doe" who is richer than god and lives in his parents' basement.

either that or a really sad 50 year old
the explosion is really a ring, not a circle!
by General Zod April 7, 2004
"Stratigic Defense Instatute". It could be some kind of super secret military organisation so secret that nobody knows the slightest damned thing about it. Or the previous description simply matches the wet dream of some guy, when in reality it's just the "company" name of some guy who made an unlicenced Star Trek board game.

Synonym: 'pathetic'.
"I am not the computer wiz! Dr.'s Bob and Geodude have that responcability. I've told you all before, It's not my yob, man!"
by Lord of the Farce April 8, 2004
Sunset District Inc/Irish. In the 70/80's a rough group of white boys. 90's and beyond...more myth than reality, but still around.
The Grove. Sunset District Inc/Irish (SDI)
by goast April 15, 2013