Definition of Insidious: Insidious (adjective): Proceeding in a gradual, subtle way, but with harmful effects; treacherous or crafty. Synonyms: stealthy, subtle, cunning, deceitful Antonyms: straightforward, obvious, honest, open Memory Tip: Think of “insidious” as something that sneaks up on you, appearing harmless at first but eventually causing harm. Conversation and Short Story Examples Here is…