The use of "bloody" as an intensifier used to be considered
highly offensive in England, as the fuss made over it in Shaw's
_Pygmalion_ shows.
the following suggested origins:
1. From an alleged Irish word _bloidhe_, meaning "rather". This
was proposed by Charles Mackay in the 19th century, but is highly
implausible: even if the word exists, it would presumably have
been pronounced /bli:/ since the early Modern Irish period. The
closest I could find to it in an Irish dictionary was _bluire_=
"a bit, some".
2. "by our Lady" (an invocation of the Virgin Mary). There *was*
an interjection "byrlady", attested since 1570 and frequently
used by Shakespeare, which *did* mean "by our Lady". But
this was an interjection, not an adverb, although a citation
from Jonathan Swift ("it grows by'r Lady cold") shows a possible
intermediate use. The transition from "byrlady" to "bloody" is
phonetically implausible.
3. "S'blood", an ancient oath shortened from "God's blood". The
Oxford Dictionary of English Etymology says this is "probably"
the origin, but the OED says "there is no ground for the notion".
The etymologies in the OED are largely untouched since the first
edition; the ODEE is generally more up to date.
4. blood with reference either to menstruation or to "the bloody
flux", an old term for dysentery. "Ingenious, but [...] much too
restricted", says Partridge.
5. "blood", an aristocratic young roisterer. The OED plumped for
this one, because its earliest citations of "bloody" as an
intensifier were in the phrase "bloody drunk", which it
conjectured meant "as drunk as a blood" (cf. "as drunk as a
lord"). But the earlier citation found by Weekley (see below)
makes this less plausible, and "bloody drunk" would be an
unusual lexicalization of "as drunk as a blood".
6. blood's being something vivid or distressing. Partridge himself
plumps for this one.
Ernest Weekley, in _Words Ancient and Modern_ (Murray, 1926),
finds analogous uses of French _sanglant_, German _blutig_, and
Dutch _bloedig_. He gives one citation that antedates those in the
OED ("A man cruelly eloquent and bluddily learned", John Marston,
1606 -- but "bluddily" may be a descriptive adverb rather than an
intensifier here), and two ("It was bloody hot walking to-day",
Swift, 1711; "bloody passionate", Samuel Richardson, 1742) that show
that "up to about 1750 it was inoffensive". He attributes the
dropping of "-ly" from "bloodily" to "an instinct which tends to
drop _-ly_ from a word already ending in _-y_", as seen in "very",
"pretty", and "jolly".
Well U did Bloody Ask...