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1.
Navy slang for when three men are having sex with a woman simultaneosly. Euphamism based on US Navy material condition of readiness which determines which fittings (doors, vents, etc.) are to be closed on a naval vessel. Condition Dog Zebra is the most secure condition, where all airtight, watertight, firetight, and fumetight fittings are secured.

So when three male sailors are teamin' a chick (whether a betty out on liberty, a female shipmate, whatever) they will "set condition dog zebra" where one guy fucks her pussy, one guy fucks her ass, and one guy is has his dick in her mouth, thus "all openings secured"
"We had some liberty in PI with this fine as ho so me and my boys set some dog zebra!"

or...

"How come all these chicks are gettin' pregnant out on WestPac? Too many people settin' dog zebra on a bitch."
by Mister Priapus June 01, 2005