From an unused root meaning to set a period (compare adah, uwd); the menstrual flux (as periodical); by implication (in plural) soiling -- filthy.
see HEBREW adah
see HEBREW uwd
H5708. ed
[עִדָּה] noun feminine menstruation, so Vrss (properly time, period); — plural absolute עִדִּים בֶּגֶד Isaiah 64:5 i.e. stained garment (figurative of best deeds of guilty people; "" טָמֵא).