In a manner relating to or according to the principles of geomancy; using geomantic methods or interpretation.
From 'geomantical' plus the adverbial suffix '-ly' (Old English '-lice'). This creates an adverb describing how an action is performed using geomantic principles.
By stacking suffixes like this, English lets us turn abstract concepts into precise descriptors—'geomantically' lets you describe exactly how someone divined using Earth patterns!
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