From an unused root meaning to set a period (compare H5710 and H5749); the menstrual flux (as periodical); by implication (in plural) soiling
KJV Usage: filthy.
Brown-Driver-Briggs' Hebrew Definitions
1. menstruation a. filthy rag, stained garment (fig. of best deeds of guilty people)
Origin: from an unused root meaning to set a period [compare H5710, H5749]
Parts of Speech: Noun Feminine