As an adjective, *solid* means firm and hard, not liquid or gas. It can also describe something reliable or strong. As a noun, it refers to a state of matter that keeps its shape.
From Middle English, from Old French *solide*, from Latin *solidus* meaning “firm, whole, dense.” The same Latin word gave us *soldier* (originally a ‘solid pay’ man) and *consolidate*.
Calling someone “solid” is like saying they’re as dependable as a rock. The physics meaning and the character meaning both come from that same idea of being firm, not easy to break or change.
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