Origin: 1350–1400; ME menstruall < L mēnstruālis having monthly courses, equiv. to mēnstru(a) monthly courses (n. use of neut. pl. of mēnstruus monthly; mēnstr- (see semester) + -uus adj. suffix, prob. on the model of annuusannual) + -ālis-al1
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.