Pernicious means causing harm in a gradual, subtle way that makes it especially dangerous. Unlike something obviously harmful, like broken glass that cuts you immediately, pernicious things damage slowly and quietly, often going unnoticed until serious harm is done.

A pernicious lie might seem small at first but spreads and causes increasing damage over time. A pernicious habit like constantly putting things off doesn't ruin your grades in one day, but steadily makes problems worse until you're overwhelmed. Scientists might describe a disease as pernicious when it weakens the body slowly, making it harder to detect and treat.

What makes something pernicious is that the harm creeps up on you gradually and deceptively. Think of how a tiny leak in a roof seems like no big deal, but over months it rots the wood and ruins the ceiling. Or how small acts of unkindness in a friendship might seem harmless individually, but gradually poison the relationship.

When you call something pernicious, you're warning that it's more dangerous than it appears. You're saying, “This looks minor, but don't be fooled. It's quietly doing real damage.”