Definition of "unquenchable"

Alas! thus ever does the weakness of our nature rebuke its strength, and genius is brought to the level—ay, below the level—of common humanity, by an unquenchable thirst for its applause.

1834, L[etitia] E[lizabeth] L[andon], Francesca Carrara. […], volume I, London: Richard Bentley, […], (successor to Henry Colburn), pages 3–4

With an unquenchable enthusiasm for locomotives and their work, at an early age I had commenced to keep engine and traffic-recording notebooks, compiled in a schoolboy's round hand.

1946 March and April, R. A. H. Weight, “Euston to the North-West”, in Railway Magazine, page 69