Good little article on the phrase origin here. Excerpt from the quoted 1838 editorial:

Ensconced in his mail of proof—for defence purely, his disposition being no ways bellicose—he snugly nestleth in his mucid bed, revels in quiescent luxury, in the unctuous loam that surroundeth him, or, with slow and dignified motion, worketh nearer the surface, as the summer suns warm the roof of his mud-palace, or sinketh deeper within, from the nipping frosts of winter.