1398, from O.Fr.
menstruel, from L.
menstrualis "monthly," especially "of or having monthly courses," from
menstruus "of menstruation, monthly," from
mensis "month" (see
menses).
Menstruation first attested 1776; O.E. equivalent was
monaðblot "month-blood." Verb
menstruate (1800) probably is a back-formation.