'arborous' originates from Latin, specifically the word 'arbor', where 'arbor' meant 'tree' and the suffix '-ous' meant 'full of' or 'having the qualities of'.

'arborous' was formed in English by combining the Latin root 'arbor' with the adjective-forming suffix '-ous' (via Medieval/early Modern English formations) and became the modern English adjective 'arborous'.

Initially, it meant 'full of trees' and this core meaning has largely remained the same in modern usage.