This word figures prominently in the song Happy Wanderer, a German import
sung by generations of American Scouts:
“I love to go a-wandering
Along the mountain track.
And as I go, I love to sing,
My knapsack on my back.”
The –sack half of the word is obvious. Americans tend to use the word backpack, but it’s the same convenient carrier.
The history of the knack- portion is a bit less obvious, but according to the Oxford English Dictionary, it seems to come from
German and/or Dutch words meaning to eat or to bite. Those words, in turn, came from an earlier word meaning to crack something or to chip a stone. The implication, therefore, is that it’s the kind of eating where you bite off small pieces of something, rather than sitting down to a sumptuous dinner.
So the knapsack was a snack sack—food on the run.
(substitute @ for AT above)