Present participle of defuse; the act of reducing danger or tension, either literally removing a fuse or figuratively calming a situation.
From 'defuse' plus the progressive suffix '-ing,' which comes from Old English and creates present participles and gerunds.
The gerund form '-ing' is unique to English among major European languages—it's the only one that lets you use verbs as nouns, so 'defusing bombs' makes defusing itself a noun, which is grammatically remarkable.
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