Concise Bible Dictionary:

The “shadowing shroud” signifies the shelter given by the spreading boughs of a great tree: such as the one to which Assyria is compared (Ezek. 31:33Behold, the Assyrian was a cedar in Lebanon with fair branches, and with a shadowing shroud, and of an high stature; and his top was among the thick boughs. (Ezekiel 31:3)).