Became solid or hardened; made something firm and definite, especially an idea or plan.
From Latin 'solidus' (firm, dense) + '-ify' (to make). The literal sense (turning to solid) came first, then was extended metaphorically to abstract concepts.
When you solidify an idea—like when a vague feeling finally becomes a concrete plan—you're using the same metaphor that physical scientists use when liquid turns to solid, showing how our brains understand abstract concepts through physical experience.
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