Etymology
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menses (n.)
"monthly discharge of blood from the uterus," 1590s, from Latin menses, plural of mensis "month" (see moon (n.)).
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*me- (2)
*mē-, Proto-Indo-European root meaning "to measure." Some words may belong instead to root *med- "to take appropriate measures."

It forms all or part of: amenorrhea; centimeter; commensurate; diameter; dimension; gematria; geometry; immense; isometric; meal (n.1) "food, time for eating;" measure; menarche; meniscus; menopause; menses; menstrual; menstruate; mensural; meter (n.1) "poetic measure;" meter (n.2) unit of length; meter (n.3) "device for measuring;" -meter; Metis; metric; metrical; metronome; -metry; Monday; month; moon; parameter; pentameter; perimeter; piecemeal; semester; symmetry; thermometer; trigonometry; trimester.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mati "measures," matra "measure;" Avestan, Old Persian ma- "to measure;" Greek metron "measure," metra "lot, portion;" Latin metri "to measure."
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menstrual (adj.)

late 14c., "pertaining to menses of females," from Old French menstruel and directly from Latin menstrualis "monthly," especially "of or having monthly courses," from menstruus "of a month, every month, monthly, pertaining to a month," from mensis "month" (see moon (n.)). Occasionally, in astronomy, "monthly" (1590s).

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menstruate (v.)

1680s, "pollute with menstrual blood" (transitive), a sense now rare or obsolete; 1752 as "to discharge the menses," probably a back-formation from menstruation, or else from Latin menstruatus, past participle of menstruare, from menstruus "monthly," from mensis "month" (see moon (n.)). Related: Menstruated; menstruating.

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