insidious

Definition

Causing harm in a stealthy, often gradual, manner.

Etymology

From Middle French insidieux, from Latin īnsidiōsus (“cunning, artful, deceitful”), from īnsidiae (“a lying in wait, an ambush, artifice, stratagem”) + -ōsus, from īnsideō (“to sit in or on”), from in (“in, on”) + sedeō (“to sit”).

Example Sentences

• "Strong and vigorous man as he looks, Livingstone has been for years the victim of a secret and insidious disease."
• "At some point in time they may become the source of an insidious cancer."
• "The nurse always must be alert to signs of slow leak or insidious infiltration."