exodus 11:5 And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts. - Free Bible Online

      5. And all the first-born in the land . . . shall die--The time, the suddenness, the dreadful severity of this coming calamity, and the peculiar description of victims, among both men and beasts, on whom it was to fall, would all contribute to aggravate its character.
      the maid-servant that is behind the mill--The grinding of the meal for daily use in every household is commonly done by female slaves and is considered the lowest employment. Two portable millstones are used for the purpose, of which the uppermost is turned by a small wooden handle, and during the operation the maid sits behind the mill.

JFB.