What does the phrase as white Mary in manger means?

The phrase "as white as Mary in the manger" is a common saying used to describe something that is very white or completely white. It likely originated from the traditional depiction of the Virgin Mary in Nativity scenes, where she is often shown dressed in white and cradling the baby Jesus in a white swaddling cloth.

The comparison implies that whatever is being described is as white as the purest white imaginable, invoking the purity and innocence associated with the Virgin Mary.

While it's not a particularly common saying in modern usage, it may be used in poetry, literature, or humorous context to emphasize the whiteness of something.