I have a website for which i wanna use this tagline "News for fools, Stuff that really matters" ...Anyway are there chances that slashdot might come behind me?
It's close enough for them to take you to court. I'm assuming they have it trademarked. That's my own opinion though. Others might think otherwise.
Slashdot is owned by OSTG. I cannot see where their slogan is trademarked. But, that does not mean they would not pursue someone who uses their concept. But, something like "News for fools. Stuff that hardly matters" will not be readily confused with "News for nerds. Stuff that matters" You should also study the slashdot site's usage policies.