Rashi frequently explained 4 and 5 letter roots by breaking them up in two 2-letter roots. For example the Hebrew root Mamzer, Mem-Mem-Zayin-Resh which means illegitimate reflects the etymology of Moom Zare, Mem-Mem Zayin-Resh which means blemished by a stranger. Todays example also illustrates this principle.

One of Esauv's descendents is called MagDeeAyl, Mem-Gimel-Daleth-Aleph-Lamed which when broken up into a two and three letter root would have the meaning of Mem-Gimel-Daleth Aleph-Lamed which means the choicest of the gods. Rashi comments This is [like] Rome [with its pantheon of Gods].

Advanced Rashi: This paragraph has evoked alot of resistance from the Bible critics who defensively attack any Biblical paragraph with fulfilled prophecy content. However Rashi did not view this paragraph as primarily historical. Rather Rashi interpreted the paragraph as follows: One of Esauv's descendants was Magdeeayl who specialized in gathering people who specialized in idolatrous gods with great power. By leading a collection of such people Magdeeayl had enormous power. This is the simple meaning of the text. However Rashi then shows the relevance of the Biblical text to modern times in Rashi's era: Rome, which specialized in a pantheon of gods, each with its own method of rulership and power, is an example of the Magdeeayl's influence.

In other words Rashi was not identifying Magdeeayl with Rome but rather identifying Magdeeayl with a world approach of which Rome was a recent current example in Rashi's time.