In a cumulative manner; by means of gathering or accumulating progressively.
From 'cumulative' (or its variants) + '-ly' adverb suffix; formed to describe the manner in which something accumulates or gathers together.
Cumulately is rarely used in modern English, but it's the most technically correct adverb form if you want to describe things happening in a step-by-step accumulative way without using the common 'cumulatively.'
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