Feminine passive participle of 'owr; something lighted, i.e. An aperture; by implication, a crevice or hole (of a serpent) -- den.
see HEBREW 'owr
H3975. muwrah
[מְאוּרָה] noun feminine light-hole — only construct מְאוּרַת — = den of great viper Isaiah 11:8, compare מָאוֺר Mishna Ohaloth 13:1 (others eye-ball ᵑ7 Ew De Di).