Wiktionary
dead asleep
a. (context idiomatic English) Sleeping in a deep sleep
Usage examples of "dead asleep".
Even though I drank the whole cup, I felt sleepy again, and as soon as I laid back on the bed I fell dead asleep.
His immersion had not harmed him, it had merely made him wholesomely drowsy, and he had been dead asleep many hours now.
Seeing her mother's face in early morning light, long and creamy next to hers on the pillow and dead asleep, her mother's fingers wiggled into the curl of her own, holding her hand.