An archaic oath or exclamation, invoking a holy thing (hall + dome) as witness; a swear word of religious nature.
From Middle English hali-dom, combining hali (holy) and dom (from Latin domus, house/thing). Originally referred to the holy rood or other sacred objects invoked as oaths.
Medieval people swore by 'hallidome' when they wanted their oath to sound extra serious—they were literally saying 'by this holy thing!'—it's like a verbal pinky swear with religious weight!
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