Word Origin & History
"eloquent and insincere rhetoric," 1915, Amer.Eng. slang. Bull in the sense of "trivial or false statements" (1914) is usually associated with this, but it existed since M.E. in the sense of "false talk, fraud," apparently from O.Fr. bole "deception, trick, scheming, intrigue," and perhaps connected
to modern Icel. bull "nonsense."
"Sais christ to ypocrites ... yee ar ... all ful with wickednes, tresun and bull." ["Cursor Mundi," c.1300]
There also was a verb bull meaning "to mock, cheat," which dates from 1530s.