Quod erat demonstrandum
Quod erat demonstrandum (normally abbreviated to Q.E.D.) is a Latin phrase that means literally "that which was to be demonstrated." In normal English, this means "thus it is proven," or "so there, nyer nyer nyer". It is used formally after the last line of a proof, and colloquially in discussion or argument in a similar fashion. Due to the scary nature of Latin, many people have been misled into thinking it actually stands for "quite easily done," which stems from math teachers who, thinking they are clever, tell this joke without identifying it as such. This ignorance is actually a godsend (or a curse), as it allows the educated to correct their cretinous peers in as condescending a manner as they feel necessary.
Poor example of usage:
- 10% of car thefts are committed by left-handed people.
- Half of all polar bears are left-handed.
- There's a 5% chance your car was stolen by a polar bear.
Never trust anyone who actually says "Q.E.D.", as they've probably just committed a logical fallacy, such as the above.