The word 'tankful' is a transparent modern English compound. Its first element, 'tank', entered English in the early 17th century from Portuguese, where 'tanque' meant a pond or reservoir, likely borrowed from a South Asian source. The suffix '-ful' is of native Germanic origin, descending from the Old English adjective 'full'. When attached to a container noun, '-ful' productively forms a unit noun indicating capacity, as seen in 'cupful', 'spoonful', and 'handful'. Thus, 'tankful' logically denotes the complete quantity required to fill a tank, following a common and intuitive pattern in English word formation.

Structure: tank (container) + ful (full of, amount that fills) - tank: Origin: Portuguese 'tanque' (reservoir, pond), perhaps from a South Asian language. Role: Root noun denoting a large container for liquid or gas. - ful: Origin: Old English '-ful', from the adjective 'full'. Role: Suffix forming nouns meaning 'the amount that fills (something)'.