In a way that involves or affects two things; with twofold character or function; in a dual manner.
From dual (adjective) + -ly (adverb suffix). This formation follows the standard English pattern of turning adjectives into adverbs by adding -ly.
A truck dually (with dual rear wheels) can carry heavier loads, and the word itself works dually—it's both an adverb for how something happens AND a truck-lover's adjective for those iconic double wheels.
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