Apparently from an unused root (probably meaning to leave interstices); A latticed window -- casement, lattice.
H822. eshnab
אֶשְׁנָב noun [masculine] window-lattice (אשנב Ecclus 42:11); Ecclus 7; absolute ׳א Judges 5:28; suffix ׳אֶשְׁנַבִּ Proverbs 7:6 (both "" חַלּוֺן).