Assumably is a word that means "presumably" or "supposedly." Its antonyms are words that suggest doubt or certainty, depending on the context of use. One such antonym is "doubtfully," which implies uncertainty or skepticism about something. Another possible antonym is "certainly," which suggests that something is assured or undeniable. A third antonym could be "impossibly," which indicates that something is not believable or cannot be done. In general, antonyms for assumably tend to convey opposing concepts of doubt or certainty, depending on the speaker's intention and the situation in question.