"Canst it" is an archaic term that means "can you do it." Since it is such an old and uncommon term, there are not many direct antonyms for it. However, if we were to simplify its meaning and look for synonyms for "ability," we could find some relevant antonyms. The most common antonyms for "ability" are "incapacity," "incompetence," "inability," "inaptitude," and "disability." While these terms do not necessarily share the same meaning as "canst it," they do provide a contrasting perspective on the notion of capability. They suggest the inability to perform a task or an action. Therefore, "canst it" can be antonymized as "cannot do it.