In a very large, impressive, or overwhelming way; to a huge degree.
From 'massive' (from Old French 'massif' meaning solid or substantial) plus the adverb suffix '-ly'. The root traces to Latin 'massa' meaning lump or mass.
When you say something is 'massively popular,' you're using a weight metaphor—the thing is so heavy with importance and scale that it overwhelms everything around it, literally or figuratively.
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