'archdruid' originates from English, specifically a compound of 'arch-' and 'druid', where 'arch-' comes from Greek 'arkhi-' meaning 'chief' and 'druid' ultimately comes from Proto-Celtic '*dru-wid-s' (see below).

'druid' changed from Proto-Celtic '*dru-wid-s' (literally 'oak-knower' from 'dru-' = 'oak' and '-wid-' = 'to know') into Old Irish 'druí' and Latinized forms such as 'druides'; 'archdruid' was formed in Modern English by combining 'arch-' with 'druid', becoming common in reference to leaders of neo-druidic orders during the 18th-century revival and later used figuratively.

Initially it meant 'the chief of the druids' (a literal leadership title within druidic orders); over time it has retained that literal sense in neo-druid contexts while also acquiring a figurative sense of 'a preeminent or senior figure' in other fields.