毒瘤 literally means 'malignant tumor', but in political and social commentary it is frequently used as a powerful metaphor for a deeply harmful, entrenched problem — such as corruption, criminal elements, or systemic dysfunction. The English word 'cancer' carries a similar figurative weight, but 毒瘤 is more specific to the imagery of a localized, malignant growth that must be surgically removed. You'll see it in phrases like 社会毒瘤 ('a cancer on society') and 铲除毒瘤 ('eradicate the malignant tumor').