'arboreal' originates from Latin, specifically the word 'arbor', where 'arbor' meant 'tree'.

'arboreal' changed from Late Latin/Medieval Latin 'arborealis' and entered English via scientific/learned Latin, eventually becoming the modern English adjective 'arboreal' and the adverb 'arboreally'.

Initially, it meant 'of or relating to trees', and over time it retained that core sense while also coming to be used specifically for organisms that live in trees ('living in trees').