Gnomish
eithog
noun. ancestor, †father, sire
eithweg
noun. ancestor, †father, sire
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A word for “ancestor” in the Gnomish Lexicon Slips of the 1910s with variants eithog or eithweg, and an original meaning of “†father, sire” (PE13/113). Tolkien indicated the variant eithog was derived from ᴱ✶attū + ōka, where the first element clearly means “father” and the second element is probably some kind of ancient masculine or agental suffix because (a) that is consistent with G. -weg in the other variant and (b) G. -og was often an agental suffix in the Gnomish Lexicon. However, it is not at all clear how ᴱ✶attū became eith-.