In a costive manner; with constipation or stinginess; reluctantly or with emotional reserve.
From 'costive' plus the adverb suffix '-ly.' Follows the standard English pattern of forming adverbs from adjectives.
Adding '-ly' to an adjective seems simple, but it lets us transform physical or emotional states into descriptions of how people behave—'he spoke costively' somehow says more than 'he was costive.'
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