In a way that is charming, attractive, or winning that causes someone to lose suspicion or defensive attitude; in a manner that makes someone lower their guard.
From 'disarming' (present participle of 'disarm,' from 'dis-' + 'arm') + '-ly' (adverb suffix). Metaphorically, to 'disarm' suspicion means to render it harmless.
This is actually a modern, frequently used word! A person can have a 'disarmingly cute smile' or speak 'disarmingly honestly'—the metaphor of removing emotional 'weapons' (suspicion, defensiveness) is so natural that this word survives in contemporary English.
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