Mid 17th century: from late Latin menstruat- 'menstruated', from the verb menstruare, from Latin menstrua 'menses'.

• month from Old English:

  A month corresponds to the period of time of the moon's revolution, and the words month and moon are related. Their ancient ancestor is also the source of Greek mēn ‘month’, from which English took menstruate (Late Middle English), menopause (late 19th century), and similar words. Shops and entertainments now open on Sundays, but in the past this was not necessarily so. Where Christianity was the dominant religion, restrictions on pleasure and activity meant that Sundays were quiet, private days. This may be behind the expression a month of Sundays, ‘a very long, seemingly endless period of time’. The expression is known from 1836 in The Clockmaker by Thomas Chandler Haliburton: ‘Mr. Slick…told him all the little girls there would fall in love with him, for they didn't see such a beautiful face once in a month of Sundays.’