Relating to or characterized by heaping or piling together; tending to accumulate or gather into masses.
From Latin 'commulare' (to heap together) plus the English suffix '-ive' meaning 'tending to.' An archaic or technical term rarely used in modern English.
This is essentially an obsolete way of saying 'cumulative'—which means that at some point in English history, people had two ways of saying the same thing, but only 'cumulative' survived.
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