One meaning of the verb 'flap' is to toss, as in tossing a pancake and a flapjack IS a sort of pancake. The 'jack' part is just an indeterminate name which we also find in apple-jack and other cases.
If anyone suggests it's an American term, they're wrong...it was in use in Britain before the Pilgrim Fathers even set sail!
I thought a Flapjack was a rolled oats and treacle, biscuit thingy.
They're generally good to eat ,though I must admit I've never tried the septic variety.
The ORIGINAL flapjack was a pancake, as described in my first answer a fortnight ago! It was first recorded as such in 1600 according to the Oxford English Dictionary, the 'bible' of English words. The biscuit definition...the American one perhaps... appears NOWHERE before the mid-1930s.
I expected the questioner to come back to us after receiving the first two such different answers, Finknottle, to tell us just WHICH variety she was referring to. After all, the biscuity (American?) version has been around for some eight decades, so these may well have been the ones in question. In other words, YOU may have been perfectly correct and I utterly wrong! I don't suppose we'll ever know now.