don't noc it till you've got the nac

Do you own a rucksack? I know you do; everyone does. I myself probably own half a dozen or so, of varying shapes and sizes and with a varying array of mud and grass adhering to the outside and a varying selection of forgotten mouldering pork pies and decomposing Peperamis hidden in the inside pockets.

"Rucksack" seems to have become the canonical term for this particular piece of equipment, with "backpack" increasingly popular as well. Back in the day, though, you had other words to choose from as well, which (though I daresay some old fogeys continue to use them) have slipped into disuse, comparatively anyway, and they are "knapsack" and "haversack". It seems very odd to me that there are three perfectly good words, all ending in "sack", which describe essentially the same thing. What's it all about?

A good dictionary is your friend here, if you want to know the derivation of words. There seems to be general agreement that the three prefixes derive as follows:

• "ruck" from the German for "back"
• "haver" from the German for "oats", the implication presumably being that that's what people used to carry in them; why you'd be lugging a bag full of oats up a hill I have no idea. Some of the dictionaries say that a haversack specifically has one strap rather than two, some disagree.
• "knap" from a German word meaning "bite" or "eat", presumably any sort of food in this case, so you're not restricted to porridge and flapjacks.

I don't know why one usage persisted while the other two now sound quaintly Famous Five-y, like they should be filled with crusty hard-boiled egg and potted meat sandwiches and fruit cake supplied by a beaming ruddy-cheeked farmer's wife who cheerily refuses any form of payment with a "be off with you, master Julian". Or is that just me?

Note also that there seems to be a trend in some quarters towards dropping the two k's from "rucksack" so it becomes "rucsac", presumably as part of a general desire for all outdoor technical equipment to be as lightweight and efficient as possible; those two cumbersome extra k's were just weighing the whole thing down. I wonder if perhaps "napsac" might make a comeback in the same way? Or "bacpac"?