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.
1823, "pertaining to health or hygiene," from French sanitaire (1812), from Latin sanitas "health," from sanus "healthy; sane" (see sane). In reference to menstrual devices, by 1881 (in sanitary towel). In U.S. history the Sanitary Commission, created by the Secretary of War in 1861, provided relief to soldiers and oversaw military lodging and hospitals. Sanitarian is by 1859 as "promoter of, or one versed in, sanitary measures or reforms;" sanitarist in that sense also is by 1859.
c. 1300, "onward movement, motion forward, a running in a prescribed direction or over a prescribed distance; path or distance prescribed for a race, a race-course" from Old French cors "course; run, running; flow of a river" (12c.), from Latin cursus "a running; a journey; direction, track navigated by a ship; flow of a stream;" from curs- past participle stem of currere "to run" (from PIE root *kers- "to run").
Also from c. 1300 as "order, sequence;" meanings "habitual or ordinary procedure" (as in course of nature) and "way of life, personal behavior or conduct" are from early 14c.
Most of the extended senses developed 14c. from notion of "line in which something moves" (as in hold one's course) or "stage through which something must pass in its progress." Thus, via the meaning "series or succession in a specified or systematized order" (mid-14c.) comes the senses of "succession of prescribed acts intended to bring about a particular result" (c. 1600, as in course of treatment) and the academic meaning "planned series of study" (c. 1600; in French from 14c.), also "that part of a meal which is served at once and separately" (late 14c.).
Meaning "the flow of a stream of water" is from mid-14c.; that of "channel in which water flows" is from 1660s. Courses was used for the flow of bodily fluids and 'humors' from late 14c.; specifically of menstrual flux from 1560s.
Adverbial phrase of course "by consequence, in regular or natural order" is attested from 1540s, literally "of the ordinary course;" earlier in the same sense was bi cours (c. 1300). Matter of course "something to be expected" is by 1739.
"of the nature of blood, pertaining to blood, bleeding, covered in blood," Old English blodig, adjective from blod (see blood). Common Germanic, compare Old Frisian blodich, Old Saxon blôdag, Dutch bloedig, Old High German bluotag, German blutig. From late 14c. as "involving bloodshed;" 1560s as "bloodthirsty, cruel, tainted with blood-crimes."
It has been a British intensive swear word at least since 1676. Weekley relates it to the purely intensive use of the cognate Dutch bloed, German Blut. But perhaps it ultimately is connected with bloods in the slang sense of "rowdy young aristocrats" (see blood (n.)) via expressions such as bloody drunk "as drunk as a blood."
Partridge reports that it was "respectable" before c. 1750, and it was used by Fielding and Swift, but heavily tabooed c. 1750-c. 1920, perhaps from imagined association with menstruation; Johnson calls it "very vulgar," and OED writes of it, "now constantly in the mouths of the lowest classes, but by respectable people considered 'a horrid word', on par with obscene or profane language."
The onset of the taboo against bloody coincides with the increase in linguistic prudery that presaged the Victorian Era but it is hard to say what the precise cause was in the case of this specific word. Attempts have been made to explain the term's extraordinary shock power by invoking etymology. Theories that derive it from such oaths as "By our Lady" or "God's blood" seem farfetched, however. More likely, the taboo stemmed from the fear that many people have of blood and, in the minds of some, from an association with menstrual bleeding. Whatever, the term was debarred from polite society during the whole of the nineteenth century. [Rawson]
Shaw shocked theatergoers when he put it in the mouth of Eliza Doolittle in "Pygmalion" (1913), and for a time the word was known euphemistically as "the Shavian adjective." It was avoided in print as late as 1936. Bloody Sunday, Jan. 30, 1972, was when 13 civilians were killed by British troops at protest in Londonderry, Northern Ireland.