More airy or having more open space; less dense or heavy in feeling or appearance.
From aery (variant of eyrie, an eagle's nest) plus comparative suffix -er, evolving to mean resembling an airy nest or open space. The sense shifted from literal high nests to describing anything light and open.
Aerier shows how English took a specific word (aery = eagle's nest) and generalized it to mean 'like a nest in the sky'—light and lofty. This kind of metaphorical expansion happens constantly: we use 'nest' to describe investment portfolios and cozy homes, all from birds.
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