In a manner involving estimation or rough calculation; in a way that suggests approximation rather than exactness.
From estimating (present participle of estimate) + -ly. This is a rare formation where the gerund/present participle takes the adverbial suffix to create an adverb meaning 'in an estimating manner.'
This word is so uncommon that most speakers would say 'approximately' or 'roughly' instead—it shows how English sometimes creates grammatically possible words that almost nobody actually uses.
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