In an unrestrained, uninhibited, or wild manner; with complete lack of self-control or constraint.
From 'abandoned' (adjective meaning unrestrained) plus the adverbial suffix '-ly.' The meaning shifted from 'having been left behind' to 'wild and uncontrolled.'
When someone acts 'abandonedly,' they're throwing caution to the wind so completely that the word almost captures recklessness as a physical force—it's the opposite of restraint.
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