Sound Asleep Meaning

(idiomatic) Sleeping still and silently.

Example: 1874, Marcus Clarke, For the Term of His Natural Life Chapter V
  When Frere had come down, an hour before, the prisoners were all snugly between their blankets. They were not so now; though, at the first clink of the bolts, they would be back again in their old positions, to all appearances sound asleep.