First Shirt. Military slang for the First Sergeant of a Military Unit. Appointed by and an assistant to the commander of a Military Unit. First Shirt is generally the highest ranked Non-Commissioned officer in the unit; although he may be any rank of sergeant depending on duty assignments. The First Shirt handles many administrative duties and minor punishments handed out to members of the unit for the commanding officer. Since the First Shirt is on daily speaking terms with the commanding officer; he will have more "power" than his rank would otherwise indicate. "See the First Shirt" generally means that you are in trouble. First Shirt is especially used in the U.S.A.F.