Yore: This is an archaic word meaning "of a former time." It's not specifically cowboy slang but can be used in a more rustic, folksy style of speaking.
You're: This is a contraction of "you are" and is not slang at all. It's standard English usage.
Could you provide more context or the full phrase where you encountered this word? That would help me understand what you are asking and give you a more accurate answer.