Fast asleep means sleeping very deeply. Many learners think fast here means quickly, but that is not the idea.
In this expression, fast keeps an older meaning: firmly, securely, or fixed in place. The same older sense appears in phrases like hold fast or stuck fast. So if someone is fast asleep, they are firmly in sleep, not easily woken.
This helps explain why the phrase sounds stronger than simply saying someone is asleep. It suggests deep, solid sleep.
- The children were fast asleep after the long trip.
- By the time I got home, she was already fast asleep.
So the key point is simple: fast asleep is about depth of sleep, not speed. It is a good example of how older meanings can stay alive inside common modern expressions.
