abbreviation of zeppelin, attested by 1915.
Examples from the Web for zep
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
I had been too poor to fly up till then, and Zep had been too little to play football.
The Zep, a failure in fact, was a splendid success in fiction.
I was crazy about aviation, and Zep was crazy about football.