the.com/shadow of a doubt

means A trace of uncertainty so small it barely qualifies as doubt at all — used mainly to insist there isn't any.

from The phrase rides on 'shadow' in its old sense of a faint, insubstantial trace rather than darkness itself — the same sense behind phrases like 'not the shadow of a chance.' It's recorded in English by the 19th century and became courtroom-famous through the legal standard 'beyond a reasonable doubt,' with 'beyond a shadow of a doubt' trailing alongside it as the more emphatic, less technical cousin — used when people want to sound legally certain without actually being lawyers.