In a firm, stable, or strong manner; thoroughly and reliably without wavering.
From solid (from Latin solidus, meaning firm or dense) plus the -ly adverb suffix, which comes from Old English -lice (in the manner of).
Words ending in -ly are how English turns adjectives into adverbs, but 'solidly' is extra useful because it works both physically (the ice froze solidly) and metaphorically (she's solidly behind the plan).
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