Green’s Dictionary of Slang

(US campus) a student who is not a member of a Greek-letter fraternity.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 22: barb, n. A non-fraternity man.
1913		Daily Nebraskan 8 Mar. in DN IV:ii 128: A group of prominent ‘barbs’ have petitioned for a chapter of Alpha Sigma Phi according to the rumor traveling the inner circles of University gossip.
1925		S. Lewis Arrowsmith 21: In college Martin had been a ‘barb’ – he had not belonged to a Greek Letter.
1927		W.R. Morse ‘Stanford Expressions’ in AS II:6 275: barb—a non-fraternity student secret society.