Relating to or characteristic of the most extreme or fundamental form of heresy; profoundly contrary to established doctrine.
From archi- (chief, extreme) + heretical (expressing views opposed to accepted doctrine). The prefix intensifies 'heretical' to indicate heresy in its most severe form.
Medieval theologians actually used 'archi-' words to rank heresies by severity—calling something 'archiheretical' was like labeling it the worst possible deviation from religious truth you could imagine.
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