If a briefcase is a leather bag in which one keeps one's briefs, and a cardcase is a sort of stiff envelope in which one keeps calling- or business-cards, and a crankcase is that part of an automobile which houses the crankshaft, and a pillowcase is a cloth bag which holds a pillow, and a suitcase is a piece of luggage in which one carries a suit, then why isn't a keepcase the thing or place in which one keeps one's keep?