A clipping of plebeian and plebe, sometimes also understood as a back-formation from plebs.
Meanings
A commoner; a member of the lower class of a society.
(derogatory) A common person; an unsophisticated or cultureless person.
You're under my roof, you pleb. 1795, John O'Keeffe, Life's Vagaries, act V, scene ii, line 85:
(US, derogatory, slang, usually) A freshman cadet at a military academy.
At Annapolis, the natives are crabs, the freshmen plebs, the sophomores youngsters. 1922, Dialect Notes, number 5, American Dialect Society, page 189:
I found some of the novices, plebs they are called, home-sick, and weary with their discipline. 1838, Caroline H. Gilman, The Poetry of Travelling in the United States..., page 76: