"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."