A member which is a good deal smaller when flaccid than when erect, so that it is protected during combat with pirates, ninjas, anyone who might try to clock you in the cock. A grow-er, rather than a show-er, which remains at a more or less constant size.
Jenny: "Er, isn't it a little... small?"
George: "Oh yeah? Battle penis, extend!"
Jenny: "Why, it's the perfect defense against pirates!"