Intrusive N in Messenger, Passenger etc.

[Josh Macfelder]

A friend of mine asked me where the N in "messenger/passenger etc"
came from, and I was only able to come up with "It's an intrusive N"
but I have no other information about this feature. Except for maybe
the fact that Merriam-Webster's traces it back to the Middle English
period.
Also, the Shorter OED that I'm using gives the following etymology:
"Old & mod. French passager use as noun of adjective meaning
'passing', formed as PASSAGE noun: see -ER2. For the intrusive n cf.
harbinger, messenger, etc."
Since it's just "compare with," any additional info would be appreciated.
Thanks in advance.

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