A discussion, esp one among good companions passing time idly but investigating important topics
[1920+ College students; perhaps influenced by bull, ''discuss, visit another student to pass the time,'' attested as college slang by 1850]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with bull session
An informal discussion, as in College students love late-night bull sessions about anything and everything, from professors to poetry to politics. This expression originally referred to an exchange of opinions and anecdotes, including stories of sexual prowess, by men, and then came to be used more broadly.
[ ; c. 1915