What is the meaning of the word insidious?

The word "insidious" refers to something that proceeds in a gradual, subtle way but with harmful effects. It often describes a situation or condition that is deceitful or treacherous, appearing harmless at first but ultimately leading to dangerous outcomes. The choice reflecting the meaning of "insidious" as "subtle" captures this essence, highlighting the stealthy and insidious nature of things that may not be immediately apparent but can lead to significant harm or trouble over time.

In contrast, "reliable" suggests something that can be depended on, "limited" indicates restrictions or boundaries, and "rapid" implies quickness or swiftness—all of which do not align with the more nuanced and deceptive implications of being insidious. Thus, "subtle" effectively embodies the true meaning of the word.