Soh-chim

From Memory Alpha, the free Star Trek reference

Soh-chim is a Klingon word which has no direct translation, but is most closely approximated by "step-sister/brother" and similar to the Human concept of a "god-parent" . This title is conferred upon a respected adult friend by a Klingon parent; it is legally binding and well-recognized in Klingon law. A soh-chim is usually named to secure a child's future well-being when his or her parent(s) go into battle and the odds are against returning.

In 2370, Worf asked Counselor Deanna Troi to be his son Alexander Rozhenko's soh-chim in case anything were to happen to him. She gladly accepted the role. (TNG: "Parallels")