[mwest]

I had the same question some time ago. The Wikipedia page is, as always, quite interesting. Although I think the tl;dr is "the name's so old, no one really knows". But the theory is:

https://en.wikipedia.org/wiki/Isle_of_Man#Name

"The name is probably cognate with the Welsh name of the island of Anglesey, Ynys Môn, usually derived from a Celtic word for 'mountain' (reflected in Welsh mynydd, Breton menez, and Scottish Gaelic monadh), from a Proto-Celtic *moniyos."

[xg15]

That's interesting indeed, thanks a lot for the pointer. So the ancient meaning might have been something like "island of the mountain".

As a retro gamer, I also appreciate that at some point the name appears to have been "isle of mana".