Let off some steam is an idiom that means to express one's emotions, usually in a loud or aggressive way. The crossword solutions for this term are VENT, HISS, and BOIL. To VENT is to express one's feelings or frustrations in a loud or forceful manner. To HISS is to make a sound like a snake, often used to express disapproval or anger. To BOIL is to become extremely angry, often to the point of boiling over. Together, these three words describe the act of letting off some steam.