Wiktionary
sound asleep
a. (context idiomatic English) Sleeping still and silently.
WordNet
sound asleep
adj. sleeping deeply; "lying fast asleep on the sofa"; "it would be cruel to wake him; he's sound asleep" [syn: fast asleep(p), sound asleep(p)]
Usage examples of "sound asleep".
The motion of the little island, as it rode lightly over the waves, was very soothing and before long the old soldier and the young prince were sound asleep too, leaving only the weather cock on guard.