insidious
in-SID-ee-usadjMeaning spreading harm slowly and secretly
Memory hook IN-SIDE-ious: harm that sneaks INSIDE. A tiny leak drips inside the wall for months where no one can see it, until the ceiling sags; the damage is insidious.
Rust is insidious: it eats away at a bridge long before anyone sees the damage.
In context The rumor was insidious; no one noticed it spreading from table to table until, weeks later, half the class had stopped speaking to Dev.
Similar stealthy, creeping, sneaky, treacherous
Opposite obvious, open, harmless
From Latin insidiosus, “cunning,” from insidiae (an ambush), from insidere (to sit in, to lie in wait).