From two Latin words, "filum," a thread, and "granum," a grain.
"Stories from Tagore"
Rabindranath Tagore
The obliquity of the nerves gradually increases, till in the lower part of the canal-from the second lumbar vertebra onward-they run parallel with the filum terminale and together constitute the cauda equina.
"Manual of Surgery Volume Second: Extremities--Head--Neck. Sixth Edition."
Alexander Miles Alexis Thomson
Ferns are termed Filices, from the Latin word filum, a thread, because of their filamentary fronds.
"Herbal Simples Approved for Modern Uses of Cure"
William Thomas Fernie