From: http://www.briss.com/bris_stuff_sandek.html

The role of Sandek is defined as the highest honor an individual can have as a participant within a bris ceremony. The origin of the term Sandek has been attributed to a derivation from the Greek word suntekos which means 'companion of child'. The main role of a Sandek is to be seated during the ceremony and hold the baby on their knees.