abbreviation of zeppelin, attested by 1915.
Online Etymology Dictionary, © 2010 Douglas Harper
Examples from the Web for zep
I was crazy about aviation, and Zep was crazy about football.
The Zep, a failure in fact, was a splendid success in fiction.
I had been too poor to fly up till then, and Zep had been too little to play football.
I am a timid youth, ansemic, wear spectacles, and am frightened by a Zep raid!The Journal of a Disappointed Man
Wilhelm Nero Pilate Barbellion