Thick-rammed is another was of saying tightly packed. The expression was used during Colonial times to describe the packing of gunpowder into cannons or other firearms. Milton uses it here as a metaphor for the forceful penetration of the rape. Milton chose "thick-rammed" instead of tightly packed because these are the key words in understanding the violence of this act. The sole purpose of "thick-rammed" is to re-emphasize the violence of this act. These words are showing a violation of sorts. Satan is forcing "these" upon the cannon (birth canal). The idea is the more violent and evil the act of ramming the gunpowder into the cannon, the more violent and evil the explosion will be.