An Etymological Dictionary of the German Language/Zäh
Zäh, adj., ‘tough, viscous, obstinate,’ from the equiv. MidHG. zœhe, OHG. zâhi, adj.; corresponding to Du. taai, AS. tôh, E. tough; Goth. *tâhu (from *tanhu-) has to be assumed. With the Teut. root tanh, ‘to hold firmly together,’ are also connected AS. getęnge, ‘close to, oppressing,’ and OSax. bitęngi, ‘pressing.’ Zange appears on account of its meaning to belong to a different root.