The word dwimmerlaik is said to mean "work of necromancy, spectre" in Rohanese.. Christina Scull and Wayne G. Hammond have suggested that dwimmerlaik derives from Middle English dweomer, Old English (gwe)dwimor, -er ("illusion, phantom") + Middle English -layk, -laik ("play"). See also Dwimordene, Dwimorberg; Saruman is also referred to as dwimmer-crafty by Éomer.
It is not clear if the word refers to a generic creature of the Rohanese folklore, such as the holbytlan or a phantom; therefore when Éowyn confronted the Witch-king she likely rather identified him with one, than coining the word on that moment.
David Day in A Tolkien Bestiary considered that a Dwimmerlaik is any creature of Rohanese superstitious folklore that includes Elves and Ents. However, other than the appearance of the root dwimor- in the abovementioned names, there is no indication in Tolkien's writings that the word pre-existed or that it was used for other races.
- J.R.R. Tolkien, The Lord of the Rings, The Return of the King, "The Battle of the Pelennor Fields"
- David Day, A Tolkien Bestiary
- J.R.R. Tolkien, The Lord of the Rings, "Index"
- Wayne G. Hammond and Christina Scull (eds), The Lord of the Rings: A Reader's Companion, p. 562