Plural of coasting; the act of traveling along a coast or moving without power (like sailing downhill on a bike without pedaling).
From 'coast' as a verb meaning to sail along or travel the coastline, plus the '-ing' suffix creating a noun. The secondary meaning of moving without effort comes from 19th-century transportation terminology.
The phrase 'coasting along' became a metaphor for doing something lazily—did you know this comes from actual ships coasting (traveling) along shores using currents and wind instead of rowing hard?
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