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).

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.