Pronounced niu (falling tone) according to "Tang Yun" (Tang Yun), and niu according to "Ji Yun" (Ji Yun), with the same pronunciation as "niu". "Shuowen Jiezi" (Shuowen Jiezi) explains it as meaning "to stab".
Also, according to "Guang Yun" (Guang Yun) and "Ji Yun" (Ji Yun), it is pronounced niu (falling tone), with the same pronunciation as "nu". The meaning is the same.