also bull shit, "eloquent and insincere rhetoric," 1914, 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)).
<a href="https://www.etymonline.com/word/BS">Etymology of BS by etymonline</a>
Harper, D. (n.d.). Etymology of BS. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/BS