'apoplex' originates from Greek, specifically the word 'apoplēxis' (ἀποπληξία), where 'apo-' meant 'away from' and 'plēssō' (from the root 'plēx-') meant 'to strike'.

'apoplēxis' passed into Latin as 'apoplexia', then into Middle English as 'apoplexie' (or 'apoplexy'), and eventually appeared in modern English in variants such as 'apoplex' (now archaic) and the more common 'apoplexy'.

Initially, it meant 'a striking away' (i.e., a sudden striking down or seizure); over time it came to denote specifically a cerebral stroke or sudden seizure, and 'apoplex' has become archaic while 'apoplexy' remains in use.