This shirt is a very country-plaid, in a western cut. It does up with little black buttons, and has light brown embroidered flowers on the shoulders (seriously, Sam? Embroidered flowers? You aren’t exactly screaming demon-Hunter here.)
In 99 Problems (5x17), Sam is wearing this shirt as he and Dean speed down the highway fleeing demons in the beginning of the episode. He takes it off sometime on the trip into Blue Earth, MN, once they are rescued. When they arrive at the church, we see Sam through the shirt to Dean, who throws it into the trunk with his own shirt.
FATE: This shit was already damaged the first time we saw it. Sam had a wound to his left bicep that was bleeding profusely through the torn sleeve.