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oso m. (Noun) "bear"
11th cent. From Latin ursus 'id.' From Proto-Italic *orsso- 'id.' From Proto-Indo-European *h2tḱo- 'id.'
Indo-European
Romance
Asturian osu, Portuguese usso, Galician oso, Catalan ós, French ours, Italian orso, Aromanian ursu, Romanian urs, Sardinian ursu
Celtic
Old Irish art "bear," "hero," Middle Welsh arth "bear," Old Breton Ard-, Arth- 'id.,' Gaulish Artio, bear goddess
Albanian
Albanian arí "bear"
Hellenic
Ancient Greek ἄρκτος (árktos) "bear"
Armenian
Armenian arǰ "bear"
Indo-Iranian
Sanskrit ŕ̥kṣa- "bear," Young Avestan arša 'id.'

As de Vaan (2014) points out, the word was heavily subjected to taboo distortion, as it was forbidden to speak the name of the bear during a hunt. Thus the word is phonologically warped. The appurtanance of Latin ursus is difficult to explain, Proto-Italic *orsso- should yield **orsus; furthermore, Proto-Italic *orsso- is difficult to explain because Proto-Indo-European *h2rtḱo- should yield **arsso-.