1966, The Hobbit (3rd edition 1966; HarperCollins Publishers, London):
(1) In English the only correct plural of dwarf is dwarfs, and the adjective is dwarfish. In this story dwarves and dwarvish are used, but only when speaking of the ancient people to whom Thorin Oakenshield and his companions belonged.
Etymology
A variant of dwarfish, coined by J. R. R. Tolkien.