Relating to, characterized by, or having the effect of accumulating; tending to gather or build up in quantity or effect over time.
From accumulate + '-ive' (suffix forming adjectives). Used since the 17th century in English to describe things with an accumulating quality or tendency.
Stress is accumulative—each small worry adds to the next, which is why people often break down after a chain of minor problems rather than one big disaster; the buildup matters more than any single event.
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