Have a big mouth is an idiom that is used to describe someone who talks too much or reveals too much information. YAWN, GAPE, and BLAB are all words that can be used to describe this behavior. Yawning is when someone opens their mouth wide and takes a deep breath. Gaping is when someone opens their mouth wide in surprise or shock. Blabbing is when someone talks too much or reveals too much information. All of these words can be used to describe someone who has a big mouth.