續一切經音義 6.8

6.8

拇指（上莫補反古今正字云拇者足大指也從手母聲也）。

拇指 (The upper character has the fanqie 莫補 [m- + -(b)u = mǔ]; the Gujin zhengzi says: 'The 拇 is the big toe of the foot; it is formed with the classifier 手 [hand] and phonetic 母').