Here we see Marshal Burks confronting Lucas. When Lucas grabs his rifle he throws his shot glass down. If you watch this scene closely it looks like his glass wasn't quite empty and some whiskey might have spilled out.
If you look on his sleeve (left picture, left arm) it looks like he got his arm in the whiskey and it left a wet spot. Later in McKee's office (right) it looks like the stain is gone. It could have dried in the meantime. But in the promo picture below you can clearly see the wet spot. This picture was shot right after the scene with the marshal and Lucas is setting with McKee and Ross. Cowgirl!
Bloopers - The Wyoming Story ― Again!
around The McCain Ranch