A mild oath or exclamation, derived from 'holy relic' and used in archaic English as a swear-by formula.
From 'holy' + 'dome' (an archaic word for 'judgment' or 'dominion'), contracted from 'by holy dome' or similar. Common in Middle and Early Modern English as a softened oath.
Medieval people couldn't swear by 'holy relics' without the church getting mad, so they created funny-sounding euphemisms like 'halidome'—it's basically the original 'gosh darn it'!
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