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"cobweb-like," especially of the membrane around the brain and spinal cord, 1789, from Modern Latin arachnoides, from Greek arakhnoeides "cobweb-like," from arakhne "cobweb" (see arachnid) + -oeides (see -oid).
arachnoid a·rach·noid (ə-rāk'noid')adj. Resembling a cobweb. Used of the arachnoid membrane covering the brain and spinal cord.