the word vaporous is a direct descendant of the latin 'vapor', meaning steam or exhalation. the suffix '-ous', entering english via old french from the latin '-osus', is a prolific adjectival suffix meaning "full of" or "characterized by". the literal construction, "full of vapor", perfectly describes a misty or steamy physical state. through semantic extension, this tangible quality of being diffuse and insubstantial was applied abstractly to describe ideas, promises, or arguments that are similarly lacking in solidity or clarity.

Structure: vapor (steam) + ous (adjective suffix) vapor (latin: steam, exhalation; functional role: root conveying the core substance). ous (latin via old french; functional role: suffix forming adjectives meaning "full of" or "having the qualities of").