secure, solid, firm, substantial / stout / sterling

solid

(geometry) solid angle (three-dimensional analog of an angle)

(geometry) solid of revolution

(simile) solid as a rock, rock solid, tough as nails

(simile) very solid, rock solid / (simile) in the pink of health, fit as a fiddle

plural of solide

firmly, solidly, securely

(figuratively) to be in a strong enough position, to be on a firm footing, to be solid enough; not to have a faint heart / (figuratively) to be in a strong enough position, to be on a firm footing, to be solid enough; not to have a faint heart; to have enough financial means to bear the costs of something, to have enough financial backing