The sandak (sandek in Yiddish) is the person who holds a baby boy on his knees during a circumcision. To act as sandak is a great honour. There is a tradition of appointing one of the child’s grandfathers as sandak (though steady hands are certainly an asset for the position).
The origin of the word is unclear. It certainly isn’t Hebrew and although it probably derives from Greek, the precise Greek source is in dispute. According to the Encylopaedia Judaica, there are two theories. The first is that the root is the word sydicos, meaning patron, which is related to the English word syndicate. The alternative is that the origin is the root synteknos, meaning companion of the father, similar to the role of godfather that has developed in Christian ceremonies.
In some Ashkenazi communities, the sandak is called the kvater which is a corruption of the German gevater, meaning godfather. In other communities, the kvater and kvaterin play a different role in the ceremony. They are a married couple, usually without children. The mother hands the baby to the kvaterin, who hands it to her husband, the kvater, who hands it to the baby’s father, who hands it to the sandek. Serving as kvater and kvaterin is meant to be auspicious for having children. It has worked for many couples!