Sandek (Hebrew: סנדק, "companion of child") is the term for a person honored at a Jewish brit milah (circumcision) ceremony, traditionally either by holding the baby boy on the knees or thighs while the mohel performs the brit milah or by handing the baby to the mohel. The origin of the term has been attributed to a derivation from the Greek sunteknos (syn meaning plus and tekno meaning child), which means "companion of child.
The Rabbi Moses Isserles (Rema) recorded the practice of the sandek holding the baby on his thighs (Yoreh Dayah 265:11). The Biur Hagra (YD 265:44) cites the Midrash Shochar Tov that explains that this is based on Psalms 35:10, which states "All my bones shall say: 'YHWH who is like you?'"Midrash outlines how every body part is used in the service of God and says that our thighs participate in the service of God by placing the baby on them during the brit.
The Rema records a custom that a father should not honour the same individual twice with being the sandek for his children. The reason is that the sandek is compared to a kohen (priest) offering the ketoret (incense offering) in the Beit HaMikdash (Jewish Temple). The procedure regarding the Ketoret is that a Kohen does not perform this Mitzva (commandment) more than once in his lifetime. God rewards the Kohen who offers the Ketoret with wealth. Thus, we want to afford the opportunity to as many Kohanim as possible to become wealthy (Babylonian Talmud Tractate Yoma 26a). Similarly, we wish to afford as many people as possible to serve as a Sandek and receive God's blessing to become wealthy.