ɪˈluːsɪv ɪnˈsɪdi.əs pəˈnɪʃəs pɚˈnɪʃəs ˈsʌt(ə)l working or spreading in a hidden and usually injurious way glaucoma is an insidious disease a subtle poison difficult to detect or grasp by the mind or analyze his whole attitude had undergone a subtle change a subtle difference that elusive thing the soul able to make fine distinctions a subtle mind