ANSWER: This is a proverbial saying for the inadequate means for the defense of Jerusalem against a strong army. The Hebrew word for "covering" is used in chap. 22:8 for outworks of defense, the barrier of the city and of the country. The Jews were as poorly prepared for a siege as a tipsy man was for sleep whom I found one evening lying on the door of my veranda trying to keep himself warm by covering himself with the rubber doormat.
— Steele's Answers p. 151, 152.