In a manner characterized by guessing or approximation rather than certainty.
From 'guessing' (present participle of guess) plus the adverbial suffix '-ly' (from Old English 'lich' meaning like/manner). Creates an adverb describing how an action is performed.
The '-ly' suffix is so standard for creating adverbs from adjectives that we rarely notice it, but 'guessingly' shows how it can also attach to gerunds (verb-forms ending in '-ing'), creating adverbs that describe manner.
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