A thick pancake; in British usage, a baked oat biscuit or cereal bar held together with syrup.
From flap (something that moves loosely) and jack (a small object or portion), originally American for pancakes, later adopting different meanings in British English.
Flapjack is a fascinating example of language divergence—American and British English kept the same word but completely changed what it means; Americans think pancake, British people think oat bar!
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