This term came about through a very popular cartoon of a helmet topping off a pair of boots to signify the veteran with just a short time left in his enlistment and keeping a REALLY low profile, turtle-like, trying to make it out alive. Among VietNam and older veterans a challenge term by which infantrymen recognized each other.
When a veteran infantryman would meet another veteran, the question would be asked: "Are you a turtle?". To which the challenged veteran would have to either reply "You bet your sweet ass I am!" or have to buy the challenging veteran a drink.