Insidious

adjective/ɪnˈsɪdi.əs/character

Definition

Gradually and subtly harmful.

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”).

Synonyms

subtle

In a sentence

Technical debt is insidious — you don't notice until it's too late.