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Etymology

The etymology most frequently repeated, because of its semantic appropriateness, is that it derives from the Celtic penn, 'mountain', 'summit': A-penn-inus, which could have been assigned during the Celtic domination of north Italy in the 4th century BC or before. The name originally applied to the north Apennines. However, historical linguists have never found a derivation with which they all agree. Wilhelm Deecke said: " its etymology is doubtful but some derive it from the Ligurian-Celtish Pen or Ben, which means mountain peak."

A large number of place names seem to reflect pen: Penarrig, Penbrynn, Pencoid, Penmon, Pentir, etc. or ben: Beanach, Benmore, Benabuird, Benan, Bencruachan, etc. In one derivation Pen/Ben is cognate with Old Irish cenn, 'head', but an original *kwen- would be required, which is typologically not found in languages that feature labio-velars. Windisch and Brugmann reconstructed Indo-European *kwi-, deriving also the Greek Pindus Mountains from the same root, but *kwen- < *kwi- is not explained by any rule. By some, English pin, as well as pen and Latin pinna or penna, 'feather' (in the sense of the horn of the quill), have been connected to the name. This view has the word originating in Latium inconsistently with the theory of the northern origin. None of these derivations are unquestionably accepted.