Used by pseudo-intellectuals at the end of arguments. Means "PWN3D N00B" but they pretend it doesn't because they're trying really hard to be intelligent.
Mr. C: 2+2=4
PI: You are wrong QED
Mr. C: Based on what
PI: You're dumb QED
Mr. C: What?
PI: Haha I won the argument QED
Mr. C: ???????
PI: QED
Used by cocky engineering I-am-smarter-than-you-and-I-know-it students upon completion of a difficult proof. To really erate the professor, add (quite easily done) afterwards.
What someone writes when insecure about his abilities as a physicist or mathematician to make themselves feel smarter than they likely are. Typically used by douchebags who think they are doing something amazing by proving a problem from a textbook. Usually follows a terribly formed proof which lacks rigor or reasoning.
Originally Latin meaning "quod erat demonstrandum" or "which was to be shown or proven", now used mainly by physics students to insult someone when something is proven wrong or false, typically with the words "Mother Fucker" added for effect.