A flower affected by canker disease, or possibly a folk name for a specific plant species.
Compound of 'canker' and 'flower', following the same pattern as cankerberry. Likely a folk or traditional English name describing a flower diseased or damaged by canker.
Many English plant names from medieval times describe what's wrong with the plant—'cankerflower' tells you at a glance that this flower tends to get sick, which would help gardeners decide whether to plant it.
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