"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)).
by 1942, from bullshit (n.). Related: Bullshitted; bullshitting.
Someone who bullshits (1930s+)
An exclamation of disbelief, derision, and contempt in retort to some proposition
Nonsense; pretentious talk; bold and deceitful absurdities; baloney: I'm afraid your theory is chiefly bullshit (1915+)
: He tried to bullshit his way out of it (1942+)