谜面

shè yī gè mí dǐ shì qí de mí yǔ shí mí miàn wéi yī bù zhǔn yòng de zì shì shén me

设一个谜底是“棋”的谜语时，谜面惟一不准用的字是什么？

In a guessing game to which the answer is chess, which word is the only one prohibited?