STORIES BEHIND THE IDIOMS Some idioms have interesting origins or are connected to the literal image behind the expression.
"Zonk" became popular slang in the 1950s, meaning to hit someone hard. The course explains that the word may come from the sound associated with hitting. "Zonk out" later describes falling asleep quickly, especially when extremely tired.
This expression was recorded in the mid-17th century. Fish need to keep their mouths open to obtain oxygen, which can make them appear to be constantly drinking.