When a extremely intoxicated person becomes intensely mad at nothing in particular, causing them to change into a pair of sweatpants and a plain t-shirt and makes a complete ass of themselves. Amusing entertainment for less drunk, more appropriately dressed party-goers.
Mike was really in sweatpants mode last night. He was wearing his sweatpants yelling at people he didn't even know for no reason. He really made himself look like a foolish asshole. It was hillarious!