'demonise' (and therefore 'demonised') originates from late 18th–19th century English use, ultimately from the Greek word 'daimon' (δαίμων) meaning 'spirit' or 'divine power', via the Late Latin/Medieval Latin element 'daemon-' and the verb-forming suffix '-ize'.

'demonise' developed from Medieval/Modern Latin 'daemonizare' and Old French influences, with English forming the verb 'demonize/demonise' (using the suffix '-ize/-ise') to mean 'treat or represent as a demon'; the modern past/p.p. form 'demonised' follows regular English verb inflection.

Initially related to the notion of a 'spirit' or 'supernatural being', the sense shifted through Christian usage to mean an 'evil spirit' or 'devil'; by modern usage the verb came to mean 'portray as evil' rather than implying literal demonic possession.