Present participle of 'enrage,' meaning making very angry, or an adjective describing something that makes people very angry.
From 'enrage' + '-ing' suffix, creating both the continuous form and an adjective.
'Enraging' is probably the most commonly used form of 'enrage' in modern English—we say 'that's enraging' far more than 'that enrages me,' showing how '-ing' forms become adjectives in casual speech.
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