Nahas writes a plain n and continues a Semitic consonant that has come down without change. The Hebrew nun, the Syriac nun and the Arabic nun are all its cognates and all keep the same name. The letter itself raises no difficulty at all.
The name does. Nahas resembles the Semitic root behind the Hebrew nachash, a serpent, closely enough for the resemblance to be noticed often, while the Northwest Semitic name nun is glossed as a fish. If the Ge'ez name really were built on the serpent word it would be an Ethiopian renaming rather than an inheritance, which would be worth knowing. But a resemblance is not a derivation. The Ge'ez letter names as a set are only partly recoverable, several of them have plainly drifted, and picking out the ones that happen to look like words a reader already knows is exactly the method that produces confident nonsense.
What can be said is that the letter is the twelfth of the fidel, that it writes n, that its cognates are secure, and that its name is not glossed in the tradition. Where that leaves a question open, the atlas records the question rather than an answer, which is the whole reason the open question exists as a kind of record here.
The open question recorded against this letter puts the case in general rather than for nahas alone: whether the Ethiopian tradition ever glossed the letter names itself, and if it did, where that gloss is written down. Until somebody has looked, a blank is the honest entry, and it is a blank with a research question attached rather than a gap nobody noticed.