'militancy' originates from English adjective 'militant', which ultimately comes from French 'militant' and Medieval Latin 'militans', where Latin 'militans' (present participle of 'militare') meant 'serving as a soldier'.

'militancy' changed from French 'militant' and Medieval Latin 'militans' and was formed in English by adding the noun-forming suffix '-cy' to create the abstract noun 'militancy'.

Initially, related forms meant 'serving as a soldier' or 'engaged in military service', but over time the meaning shifted to 'combative or aggressive support (for a cause)', reflected in the modern sense of 'militancy'.