'williamsii' originates from Modern Latin, specifically the genitive form of the proper name 'Williams', where the suffix '-ii' marks the genitive ("of") for a personal name.

'williamsii' was formed by Latinizing the English surname 'Williams' (a patronymic derived from the given name 'William') into a taxonomic epithet with the genitive ending '-ii'. The given name 'William' comes via Old French/Norman forms (e.g. 'Willelmus'/'Guillaume') from the Germanic elements 'wil' (will, desire) + 'helm' (helmet, protection).

Initially it meant 'of Williams' (belonging to or associated with a person named Williams) and this meaning is preserved in modern taxonomic usage.