One of the more idiotic etymologies regularly given by linguistic experts is the one for the usual Celtoid word for “low”. This word appears as izel in Breton, as isel in Welsh and as ísel in Old Irish. (A Celtic (i.e. “Gaulish”) cognate has apparently not yet been found, but it would most certainly have been *īsselos/*ītselos.)
The immediate origin for these words is obviously Proto-Celtic *ītselos. But beyond this, the linguistic experts imagine that this Proto-Celtic *ītselos is derived from the Proto-Indo-European locative plural of the word for “foot” – *pōts – which would have been *petsu and which meant “at the feet” (in which case the Proto-Celtic would presumably have been *φītselos).
This etymology is not entirely devoid of sense, but I consider it quite ridiculous. It’s really just silly, not to mention phonetically impossible – there is no way that a long i can result from a reflex of *petsu.
The important cognate of Proto-Celtic *ītselos that the linguistic experts routinely disregard is Latin īnferus “lower” (the source of English inferior). Latin īnferus is always considered to be a cognate of Proto-Germanic *underaz (> English under) and Sanskrit adhara, but these terms are from an original *h1ṇdher-, a form that cannot have produced the long ī in the Latin word, nor can it have produced the medial f. The exact reflex of PIE *h1ṇdher– in Latin would have been **enderus.
[By the way, the Celtic language (usually called “Gaulish”) had an exact cognate of Proto-Germanic *underaz and Sanskrit adhara, this appearing as anderon on the inscribed lead tablet found at Chamalières. A related Celtic andernados appears on the inscribed lead tablet found at Larzac. Celtic anderos possibly meant “under” or “nether”, although I consider it equally possible that it meant “original” or “primeval”…]
The existence of f in the middle of a non-compound Latin word is very peculiar, and it has had a few people scratching their heads. The f sound that appears at the beginning of many Latin words usually comes from original bh and dh, but these voiced aspirates normally became b and d in the middle of Latin words. For this reason, a hypothetical Pre-Proto-Italic *īndheros would normally have become **īnderus in Latin rather than īnferus. So there was something special in the proto-form of this word that caused the f sound to develop in medial position.
In my view, this peculiar medial f sound is the result of a dh being immediately followed by an s, as in a possible Pre-Proto-Italic *īndh-s-eros. In my opinion, the following s influenced the development of f from dh in a medial position. And this Pre-Proto-Italic *īndh-s-eros would nearly correspond to a possible Pre-Proto-Celtic *īndh-s-elos which would have become Proto-Celtic *īntselos, and which would have lost the n at some point for some reason, yielding Proto-Celtic *ītselos.
Of course, the *īndh– element in the possible Pre-Proto-Celtic *īndh-s-elos and Pre-Proto-Italic *īndh-s-eros would be related to the *h1ṇdh– element in the *h1ṇdher– that became English under and Sanskrit adhara. But the existence of the long i in the former must necessarily indicate two different forms.