going straight forward
驀 centers on moving forward directly and resolutely, like a horse charging straight ahead without deviation.
驀 combines 莫 (possibly used phonetically) with 馬 (horse), suggesting a horse moving forward. The exact historical development is uncertain.
A horse (馬) under a canopy of grass and sun (莫) charges straight ahead without stopping.
For バク, imagine a horse charging forward with a loud 'back' of hooves: back -> バク, as it goes straight forward.
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