IPA: /ˈdriːm.ləs.li/
KK: /ˈdrimləsli/
In a manner that does not involve dreams or dreaming.
She slept dreamlessly through the night, without any dreams at all.
Dreamlessly → It is formed from "dream" (from Old English *drēam*, meaning a series of thoughts, images, or emotions occurring during sleep) and "-less" (meaning without). The word "dreamlessly" means without dreams or the absence of dreams during sleep.
Think of being 'without' ('-less') any 'thoughts or images' ('dream') while you sleep — that's what dreamlessly means.