"of or pertaining to elves," 1590s, from elf; first found in Spenser, who also used it as a noun and might have been thinking of elven but the word also is a proper name in the Arthurian romances (Elphin).
thunderbolts quivered with elfin flares of heat lightning
she was an elfin creature--graceful and delicate
obsessed by things elfin and small
all the little creatures joined in the elfin dance
perpetrated a practical joke with elfin delight