OED says; motionlessly adv. ) + -ly] without motion, stilly; in a motionless manner. 1887 W.
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These posts are really old and I'm not a native speaker, but after reading them I felt I wanted to contribute something.
Why would a person not be capable of "lying motionlessly", let's say "the boy lies motionlessly behind the bush, hidden, waiting for the seeker", to go with the quote above. And I don't mean the action of lying down (how someone would do that motionlessly is beyond me)