Almost every Cyrillic letter name is an ordinary Slavonic word whose meaning is plain: I, letters, I know, word, good, it is, live, very, earth, who, how, people, think, ours, he, rest, say, word, firm, learning. The list is coherent enough that its exceptions stand out, and fert is the clearest of them.
No accepted etymology exists. Proposals have been made from Greek, from Slavonic and from other directions, and none has carried the field. The letter kher, which follows shortly after, is in a similar position, and the two are regularly discussed together because they are the pair of names that resist the pattern the rest of the alphabet follows so consistently.
Something can be said about why. Both letters write sounds that are not native to Slavonic and occur only in borrowed vocabulary. A naming scheme that works by choosing a common word beginning with the right sound has nothing to work with when the language contains no common word beginning with that sound, so whoever named these letters had to do something else, and what they did is no longer recoverable.
That is a real gap and not a small one, because the letter names are among the best evidence there is for how the alphabet was taught and what its makers were thinking. The atlas records it as an open question rather than choosing among the proposals, and the discipline that would settle it is historical Slavic lexicology working on the earliest alphabet lists.