From the late Roman cognomen Ammianus, which is of uncertain origin and meaning, but it is noteworthy that all of the bearers known to history are of Byzantine Greek descent. As such, the original form of the name must have been Ἀμμιανός (Ammianos), which thus means that Ammianus is a latinization of the original name.However, the fact that all known bearers are of Byzantine Greek descent, does not necessarily mean that the meaning and origin of the name is the same for all of them. One must remember that while Greek was the official national language in the Byzantine empire, many other languages were spoken in the empire, as there were many different ethnicities present in the empire (i.e. the empire was multi-ethnic). Therefore, bearers of the name (and their parents) were exposed to languages other than Greek during their lives and probably even had a passable command of at least one of them - in fact, it could even be that one of those languages was their actual mother tongue and Greek just the language that they spoke in more formal settings.As a result, it is possible that for at least one bearer, the name is derived from the Greek noun ἀμμά (amma), which can be a diminutive for "mother" (in which case it is the Greek equivalent of English "mommy") as well as mean "nurse". The use of this word as the basis for a name was perhaps in reference to one of the so-called "desert mothers", each of whom were initially affectionately referred to as amma "mommy", but over time, the word eventually transformed into a term that was used to denote a spiritual mother, and even later, an abbess. If this particular reference was indeed the case with at least one bearer of the name Ammianus, then it points to the fact that he and his parents were christians. Perhaps the early 4th-century saint Ammianus was an example of this, though he was born (or at least lived) in a village near Nicomedia in Bithynia (which is nowadays İzmit in Turkey).Meanwhile, for other bearers, the name could be derived from a word in one of the other languages that were spoken in the empire at the time, such as Coptic Egyptian, Hebrew, Old Arabic, Punic and Syriac. Seeing as most other recorded given names starting with Am(m)i- have been verified to be Semitic in origin, the last four of the aforementioned languages are probably among the most likely candidates, as they are all Semitic languages. As such, you might want to compare Ammianus with the Arabic name AMMAR and the Hebrew name AMMIEL, for example. However, it is also worthwhile to take a look at the non-Semitic name AMMON, which is the hellenized form of the Egyptian name Yamanu.In summary, it is difficult to pinpoint a single, exact meaning and origin to the name Ammianus. The only thing that we can say for certain about the name, is that there are no records of it before the 2nd century AD (which is when we first hear about an epigrammatist with that name, who was Greek, but probably a Roman by birth) and that in this late Graeco-Roman period, the name was very rare in the geographic region that is nowadays known as Italy.The best known bearer of this name is Ammianus Marcellinus (4th century AD), a pagan Roman soldier and historian of Byzantine Greek descent who was born in either Syria or Phoenicia.