OPRINDELSE fra latin taxus, muligvis beslægtet med græsk toxon 'bue' med tanke på træets smidighed https://ordnet.dk/ddo/ordbog?query=taks&select=taks :)

The idea of a non-indoeuropean origin of many words is surely important. "Person" has Etruscan origins but who knows were they took from. So could be especially for natural features. Best;

There is no etymological connection between Gaelic "loch" and your putative "lan", "loch" goes back to PIE *lokus (like Italian "lago"). So, I dispute both your conclusions: "alnus" is not Celtic, and there is no element meaning "marsh" in "Mediolano/Medhelan".

@Bruce, I think you got more than you bargained for! @A⚚P ☾☼ [D3], no super powers, I'm afraid, I'm just attracted by discussions on plant name etymologies, and then when you brought Celtic into it, it was pane per i miei denti! I don't mean to say that there can be no connection between "brown" and tree names, it's just in the case of Celtic "eburo-" the claim seems to have no basis. But "el-" = "red, brown" .... > "alnus", on the other hand, as far as I know seems to be quite widely accepted (and the Celtic etymology seems highly spurious), though several respectable Indo-Europeanists suggest that the root that gave "alnus" and "alder" was originally borrowed into the Italic, Germanic and Slavic branches from a non-Indo-European substrate language. I agree that it seems strange for a minor river in northern Italy to have a Greek name (I read that an Akkadian etymology has also been suggested, which is quite bizarre), but I haven't come across any convincing Celtic etymologies, which is not to say that one doesn't exist (there's also the distinct possibility that the name is pre-Celtic, since river names tend to be very conservative). Celtic "landâ" seems to mean a circumscribed area of land (probably with a specific purpose), rather than a plain. Another commonly cited etymology for the second part of the name is "lânon" which should be related to the Latin "planum" meaning a plain (flat land), but I don't think the term is widely attested in Celtic as a toponym (I only know it from this toponym -- in Insular Celtic its reflex means "blade") and if that is indeed the word then it might just be the Latin word pronounced with a Celtic accent (the Celts dropped their p's), and if, as you say, the area was a lake or swamp rather than a plain, then that etymology seems less likely. In that case the interpretation "central sanctuary" (*mediolanda) seems the most plausible.

Thank you, Coligno. A very interesting discussion!