Comprehensive resource for understanding and mastering English idioms
The idiom "zonk out" means to fall asleep suddenly and deeply, usually when one is tired or bored. It can also refer to losing consciousness due to injury or illness. The figurative meaning of the idiom is to become extremely tired or unresponsive.
The idiom "zonk out" is believed to have originated from the early days of cinema. When films were silent, audiences would watch them on small screens in dimly lit theaters. If the film was too boring or poorly made, the audience would become restless and fall asleep. They would "zone out" during the movie. The idiom has been used since the 1920s and is still commonly used today.