means Firm and stable in shape, keeping its form rather than flowing or scattering — by extension, dependable, well-made, or unbroken.

from From Latin 'solidus,' meaning firm, whole, dense, or complete — the same word that named a sturdy Roman gold coin (the 'solidus,' ancestor of the Italian 'soldo' and even the British 'shilling' abbreviation 's.'). It reached English through Old French 'solide.' The sense of 'reliable' or 'a solid friend' is a natural figurative stretch: something whole and unshakeable, with no gaps to give way.