Harmful in a slow, secret, or gradual way that is not easy to notice at first.
From Latin 'insidiosus' meaning 'deceitful, treacherous', from 'insidiae' (an ambush, plot), related to 'sedere' (to sit), as in sitting in wait. The sense of hidden danger carried over directly into English.
What makes something insidious isn’t just that it’s bad—it’s that it sneaks under your radar while you feel safe. Many modern problems, like addictive apps or slow health issues, are insidious because they feel harmless in the moment.
“Insidious” has been used in public discourse to describe perceived threats, sometimes including women’s independence or feminist movements, casting them as covertly dangerous. It has also described subtle forms of sexism that operate gradually and invisibly.
Use “insidious” carefully, reserving it for genuinely harmful, gradual processes (such as systemic bias), and avoid demonizing movements for gender equality with this term. Be specific about the harm and mechanisms involved.
["subtle but harmful","gradual and damaging","covertly damaging"]
When describing insidious forms of sexism, credit women researchers and activists who identified and named these patterns, such as work on microaggressions and implicit bias.
Complete word intelligence in one call. Free tier — 50 lookups/day.