Name of the first ruling steward of Gondor (LotR/670, 1039). Tolkien translated the name as “devoted to the house” (Let/386). It is a combination of mar(da) “house” and the suffix -(n)dil meaning “devoted to, -friend” (SA/bar, (n)dil).
Possible Etymology: In notes from the 1960s (PE17/106-7), Tolkien distinguished the word mar(da) (mard-) “house” and the word már “home” often used of the names of lands. The name Mardil might contain either, but I think marda “house” is more likely. Some authors have suggested that the “House” in this name may refer to the House (family) of the Kings of Gondor (QQ/már), but in Tolkien’s notes he said that these words were not used in this way (PE17/107).
Conceptual Development: This name first appeared in Lord of the Rings drafts from the 1940s as ᴹQ. Máraher the “Good Steward” but was quickly changed to ᴹQ. Mardil (WR/153).
Mardil masc. name, "(one) devoted to the house", sc. the "house" of the kings (Appendix A; interpreted in Letters:386). This indicates that the first element can mean "house" in the sense of family or household (see mar, már). This Mardil is described as a good steward, possibly suggesting that mardil ("one devoted to the house/family") could itself function as a common noun "(faithful) steward".