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"; the Latin equivalent of a mic drop. It is used formally after the last line of a mathematics proof, and colloquially in discussion or argument in a similar fashion.

Confusion
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, alternatively, a curse), as it allows the educated to correct their cretinous peers in as condescending a manner as they feel necessary.

As a fallacy
While fancily ending a syllogism of one's proposal with "Q.E.D." totally sounds like something a college dean — learned at the topic at hand — might do, doing so is actually not the least bit indicative of said argument being logically acceptable.

Here's an example of the "Q.E.D. stamp" being applied to a blatantly fallacious argument:

As such, treat unexamined claims of Q.E.D for what they most likely are — mere pretentious Latin shorthand for shouting "Nailed it!!" while touring the room for high-fives.