The phrase “as white as Mary in the manger” is a variation of expressions that use “white” to describe something pure, innocent, or untainted, often in the context of religious imagery. It likely references the Virgin Mary, who is often depicted as pure and immaculate in Christian tradition, particularly in depictions of her with the infant Jesus in the manger.
The use of “white” here would symbolize purity, holiness, or perhaps even the snowy, peaceful setting of the nativity scene. However, it’s not a widely recognized or traditional phrase, so its meaning might vary depending on the context in which it is used.