Top bamboo radical ⺮, the compressed top-form of
竹. Originally
笨 referred to the white pulpy core inside a bamboo stem — soft, plain, and unremarkable — which is how it picked up the 'dull, stupid' sense. Same radical groups other bamboo-built things:
笔 (brush),
笑 (laugh, originally a swaying bamboo),
篮 (basket).
