In a manner lacking bones; with flexibility or without rigid structure, often used figuratively to describe movement or character traits.
From boneless (bone + -less suffix meaning 'without') + -ly adverb marker. The figurative sense developed to describe graceful, fluid movements or weak-willed behavior.
When someone moves bonelessly, they're compared to animals like octopuses or snakes—biology gave us the image, but poets used it first to describe human movement that seems impossibly fluid.
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