What does TURTLEBACK stand for?

What does TURTLEBACK mean? We have 2 definitions. Read most used TURTLEBACK meanings below.

TURTLEBACK HUMVEE — a Humvee that does not have an open back. Usually a M1114 or M1025, when viewed from the side, looks like a turtle.
TURTLEBACK — Swimming on your back while watching the direction you came from and typically a compass-board. Usually done with a chest-mounted breathing aparatus like a Draeger.