From Latin 'solidus' (firm, whole) — the root behind soldier (paid in solidi), solder (to make solid), and solidarity.
Firm and stable in shape; not hollow or liquid.
From Old French 'solide,' from Latin 'solidus' meaning 'firm, whole, complete, entire,' from PIE *sol-ido- meaning 'whole.' The Roman gold coin 'solidus' gives us 'soldier' (one paid in solidi) and 'sou.' Key roots: solidus (Latin: "firm, whole").