I know, now. A knap can be the crest of a hill (though this dialect word is never used here in Hertfordshire) or knap can mean to hit, hammer or chip something.
The hammering knap is used a lot in Hertfordshire because the ground is full of flints. Indeed, in parts of our garden you don't even try to push your fork into the ground, you wiggle it in (if you can) between all the knobbly bits of stone.
Their abundance means that flints are used a lot for building, even though they naturally come in amoeba-like shapes that have to be knapped - that is, have the bulgy bits knocked off - if you don't want the wind whistling in and blowing out the candles.
The other local knap is knapweed, rather a lovely flower that copes very well with poor soil full of stones.
So there we are: three sorts of knap. Unfortunately none of them have anything to do with the knap of knapsack, but, hey, I'm sure all the wondering is good for the synapses of the brain.
Or perhaps it just encourages them to run screaming round in circles before screeching to a halt in utter bewilderment.
That would explain a lot.
Word To Use Today: knap. The knap of knapweed comes from the Dutch cnoppe, bud; the knap meaning hill comes from the Old English chæpp, meaning top; the chipping-off-stone meaning might be something to do with the Dutch knappen, to crack; and the knap of knapsack probably comes from the Low German knappen to bite or snap.