Spreading disease or corruption; progressively decaying or damaging, like canker disease.
Present participle of 'canker' used as an adjective. Describes the ongoing, progressive nature of canker's destructive process.
The '-ing' form captures the sense of something actively happening—not just 'diseased' but 'actively diseasing'—which is why poets liked using 'cankering' to describe slow moral decline or spreading social problems.
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