Common knowledge? Let’s have a quick look.
The AHD lexicographers seem to disagree to some extent with the usage panel.
Here is what the lexicographers say:
hoi pol•loi (hoi′ pə-loi)
The common people; the masses.
[Greek, the many : hoi, nominative pl. of ho, the; see so- in the Appendix of Indo-European roots + polloi, nominative pl. of polus, many; see pelə-1 in the Appendix of Indo-European roots.]
Note that they tell us that it means “the many” in the original Greek. Their first English definition is the common people, followed by the masses.
Given that variation of meanings among the professional staff and the usage board, I wouldn’t go so far as to call the statement of Greek-English equivalence “common knowledge”.
Random House Unabridged offers this:
the common people; the masses
Collins English Dictionary gives a British English view that similarly disagrees with AHD’s usage panel:
(often derogatory) the masses; common people
Greek, literally: the many
American Heritage, in another publication, adds yet another meaning:
The masses, the ordinary folk; the phrase is often used in a derogatory way to refer to a popular preference or incorrect opinion: From Greek, meaning“ the many.”
The American Heritage® New Dictionary of Cultural Literacy
Oxford Dictionaries online agrees:
Mid 17th century: Greek, literally ‘the many’.
Cambridge has a different definition entirely:
UK /ˌhɔɪ pəˈlɔɪ/US /ˌhɔɪ pəˈlɔɪ/ disapproving or humorous
Moving back to my side of the puddle, Merriam-Webster’s notion of the term is:
Definition of hoi polloi
1. : the general populace : masses
They offer a usage comment that begins, “In Greek, hoi polloi means simply “the many”.
Summing up, a little digging suggests that “common knowledge” is more like a scatter diagram than a single point.
There is uniformity in regard to the original Greek: “the many”. Right there we have undone the AHD usage panel’s assertion
about the term “properly” meaning the same thing in both Greek and English.