'attackable' is formed from the verb 'attack' + the adjectival suffix '-able'. 'Attack' comes from Old French 'ataquer' (or Middle French 'attaquer'), and the suffix '-able' comes from Latin '-abilis' via Old French.

'attack' entered English in Middle English as 'attaken/attaken' from Old French 'ataquer' (from Vulgar/Medieval Latin influence and Romance verbs like Italian 'attaccare'), and later combined with the productive suffix '-able' (from Latin '-abilis' through Old French) to form 'attackable' in Modern English.

Originally the root verb meant 'to set upon, to seize or assail', and the derived adjective 'attackable' came to mean 'able to be attacked' or more generally 'vulnerable'; the core sense of 'assailability' has been retained.