S. atheg n. “[little] father; thumb” (Category: Father)
A play-name for thumb, as well as an affection word for “father”, in notes Elvish Hands, Fingers and Numerals from the late 1960s (VT48/6). It is a combination of the root √AT with the diminutive suffix -eg¹. Tolkien considered but rejected several alternate forms including aderig and tadeg (VT48/17 note #13).
References ✧ VT48/6, 17
|adar||“father”||✧ VT48/17 (adar)|