'many-haired' originates from Modern English compounding of 'many' and 'haired', where 'many' meant 'a large number' and 'hair' meant 'hair'.

'many' comes from Old English 'manig' meaning 'many', and 'hair' comes from Old English 'hær'; the adjectival form was formed in Modern English by compounding 'many' with the adjectival/past-participial suffix '-ed' on 'hair' to produce 'many-haired'.

Initially it meant 'having a large number of hairs', and this basic meaning has remained stable into modern usage.