An aircraft without an engine that flies by riding air currents, or a porch swing that moves back and forth.
From 'glide' plus suffix '-er'. 'Glide' comes from Old English 'glidan' meaning to move smoothly. The furniture meaning developed from the smooth back-and-forth motion.
Gliders represent humanity's closest approach to bird-like flight - they must read the invisible rivers of rising air the same way ancient mariners read ocean currents, turning the sky into a navigable sea.
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