Back in our college days a “bull session” was a gathering of young men, always men and always young, congregated informally in some dormitory room over late coffee or beer or other refreshments, with conversation ranging over any topic or topics that might be argued or discussed.
The topics might be religion, giving rise to a diversity of views or half-formed ideas, or sex, or music, art, literature, or anything discussable, but none of the participants in such a session certainly ever considered these sessions, sometimes, though rarely, including a college prof or instructor, to consist of “foolish talk, stuff, claptrap,” in the words of a recent dictionary.
The discussants took them seriously, no matter what others may have thought, though, of course, frivolity did enter now and then.
But along about that time “bull” degenerated into another sense, euphemistically often called “bushwa,” what one might describe as the end product of the domestic bull, used chiefly as fertilizer; its slang sense was idle talk, stuff, nonsense, claptrap.
The “bull session” of earlier days then descended into any confabulation, male or female, devoted to such chatter, and “to shoot the bull” did then become hyperbole for “to talk nonsense” or, sometimes, “to brag,” or “to cheat or defraud.”