The Online Etymology dictionary describes Bullsh*t thus:
"eloquent and insincere rhetoric," 1915, American English slang; see bull (n.1) + shit (n.), probably because it smells. But bull in the sense of "trivial or false statements" (1914), which usually is associated with this, might be a continuation of Middle English bull "false talk, fraud" (see bull (n.3)).
"insincere, trifling, or deceptive talk," 1914. Popularly associated with roughly contemporary bullshit (n.) in the same sense, and in modern use often felt as a shortened form of it. There seems to have been an identical Middle English word meaning "false talk, fraud," apparently from Old French bole "deception, trick, scheming, intrigue," and perhaps related to modern Icelandic bull "nonsense."
Sais christ to ypocrites ... yee ar ... al ful wit wickednes, tresun, and bull. ["Cursor Mundi," Northumbrian, early 14c.]
There also was an early Modern English verb bull meaning "to mock, cheat," which dates from 1530s. Bull session is attested from 1920.
Also of uncertain connection with the bull that means "a gross inconsistency in language, a ludicrous blunder involving a contradiction in terms" (1630s), said by the English to be characteristic of the Irish, and thus often called an Irish bull. Sydney Smith defined it as "an apparent congruity, and real incongruity of ideas, suddenly discovered." Three examples attributed to Sir Boyle Roche: "Why should we do anything for posterity, for what, in the name of goodness, has posterity done for us?" ... "It would surely be better, Mr. Speaker, to give up not only a part, but, if necessary, even the whole of our Constitution, to preserve the remainder." ... "The best way to avoid danger is to meet it plump."
The Routledge dictionary of modern American slang and unconventional English appears to confirm the 1914 meaning of the word (pp. 140 & 141)
Further interesting observations about the 1914 reference appear in this wordwizard discussion:
Most dictionaries say that ‘bullshit, utter nonsense, a flagrant lie, flattery, insincere talk is an American expletive first recorded in 1914, but many etymologist feel that it was in use at least a century before that. The ubiquitous 1914 date appeared in the supplement to the 1972 OED (repeated by almost all dictionaries) – a pretty late appearance for such a widely used (by then) term. A quotation from a 1914 letter James Joyce wrote to Ezra Pound read: “I enclose a prize example of bullshit.” A 1915 letter of Joyce to Pound read: “ [T.S.] Eliot has sent me Bullshit & the Ballad for Big Louise. They are excellent bits of scholarly ribaldry.”
The Strong Language blog has also carried out some research about the term in their article "When “shit” hits the newspapers".
This mentalfloss article discusses 19 Old-Timey ways to call B.S.
The blog Escaping from Bullshit covers the term almost exclusively