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1.
An individual afflicted with a particular, highly specialized learning disability which prevents them from acquiring a working knowledge of certain American English grammar* rules pertaining to the proper use of the words 'your' and 'you're' in the context of written communications.

Conjugation
1. possesive: Yourtard
2. plural: Urtards
3. adjective: You're-tarded

Etymology

1. You're yoor; unstressed yer contraction** of you are: "You're certain that's right?" > Can be confused: yore, your, you're.

2. Retard: ree-tahrd a person who is stupid, obtuse, or ineffective in some way: a hopeless social retard.*

Notes
Contraction - A shortened form of a word or group of words, with the omitted letters often replaced in written English by an apostrophe, as e'er for ever, isn't for is not, dep't for department.
>> Usage: Contractions such as you're, isn't, couldn't, can't, weren't, he'll and they're are symptomatic of informal speech and writing. They are common in personal letters, business letters, journalism, and fiction; they are rare in scientific and scholarly writing. Contractions occur in formal writing mainly as representations of speech.

** specifically the rules surrounding morphological differentiations between homophonic contractions vs. possessive phonemes

See - Mac Lethal's epic Yortard dis-rap: y outube dot com / watch?v=32p8d6OudgU
Yortard - noun

A typical Yortard might write, "YOUR A DEAD MAN!"

Then, if you were to ask why, they might reply,

"Cuz my dad found YOU'RE autographed first edition

of the MLA Rubric under my mom's pillow AGAIN!

Your gonna get you're ass beat if you come to...

You'reba Linda!"
by N8, Ideal, Duke Hollister June 15, 2013