In the ancient monastic sources (Apophthegmata Patrum, Historia Monachrum In Aegypto, Palladius Historia Lausiaca, etc.), the name of a monk is generally preceded by the Greek word abbas or abba, the cognate of the Coptic apa (Sahidic) or abba (Bohairic), the Arabic aba or anba. This is clearly the Aramaic and Syriac ab in its emphatic state, abba, "father." (It is also attested, less frequently, in the feminine, in the form ammas or amma, corresponding to the Syriac ‘em, ‘emma, "mother.")
The term is applied to the monk considered as a spiritual father, because endowed with charismata, as is shown by the apothegm Antony 31 in the Apophthegmata Patrum (85 B). One day, Antony receives a letter from the emperor Constantius II inviting him to come to Constantinople. "Must I go?" Antony asks his disciple Paul (probably Paul The Simple). The latter replies, "If you go, you will be called Antony, but if you do not go, Abba Antony." If the monk remains in the desert, he is abba; but if he goes into the world, he loses his qualities as abba, that is, his charisma and spiritual authority. The story of Paul the Simple as it is related by Palladius (1904, Vol. 2, pp. 69-74) is also revealing in this respect: so long as Paul is a novice, he is simply "Paul," but when he has become a genuine ascetic, capable of working miracles, Antony calls him "Abba Paul."
This quality is not necessarily linked to a person's age. Thus, we read in the Apophthegmata Patrum (Poemen 61, 336 D) that the abbot Joseph is astonished to hear the abbot Poemen call the young Agathon abba: "He is still young; why do you call him abba?" Poemen replies, "Because his mouth [that is, his words] makes one call him abba." This quality is also independent of any function in the community. In Chapter 34 of the Historia lausiaca (1904, Vol. 2, pp. 98-100), Palladius tells the story of a nun who, out of humility, passed herself off in a community of virgins as simpleminded and thus had contempt and ill treatment heaped upon her by her companions. One day a venerable anchorite came to visit them and asked to see this nun, whose holiness had been revealed to him in a vision. When she appeared, he said to them all, "It is you who are simpleminded, for she is our amma, yours and mine." Palladius adds, "It is in fact thus that those who are spiritual are called."
However, the usage became established? very rapidly it seems? of calling every monk abba and every nun amma. Furthermore, when a monk comes to ask for a "word," or counsel, from an older monk or one whom he holds spiritually superior to himself, he addresses him deferentially as "Father." According to the Coptic lives of Pachomius, it seems that in the early Pachomian community, Pachomius was designated simply by the word Apa used as a proper noun (Lefort, 1943, pp. 375-76). Later, when the use of the word had become widespread and was applied to any monk, it was usual to designate the founder and superior of the community by saying "our father Pachomius" or "our father Apa Pachomius," and likewise for his successors, as in "our father Theodorus."
How can we explain this use of a word of Aramaic origin among the monks of Egypt? Following Reitzenstein (1916, p. 210), some have thought that it was due to an influence from Syria and the Syriac language. But we have no proof for any influence of Syrian monasticism on Coptic monasticism; moreover, the use of the word abba as applied to the monks does not seem to be of Syriac origin. In that language it only appears in translations of Greek works, notably the Apophthegmata Patrum, in the Paradise of Enanisho (seventh century), and the word was retained thereafter largely to designate the superiors of monasteries. Thus, it appears more probable, as Hausherr asserts (1955, pp. 17-39), that this monastic use of the word abba originates in the Scriptures, or more exactly the New Testament. As in Mark 14:36, Romans 8:15, and Galatians 4:6, the word was probably employed at first in the vocative, with a nuance at once of respect and of familiarity. In the New Testament the term (examined by Jeremias, 1966) is applied solely to God. The use that was made of it among the Egyptian monks is explained by the fact that the "spiritual father" was judged fit to direct or to counsel solely by virtue of the charisma that he had received from God. In a certain manner he participated in the divine paternity.

Cotelier, J. B., ed. Apophthegmata Patrum, cols. 71A-440D. PG. 65
Hausherr, I. Direction spirituelle en Orient autrefois. Orientalia Christiana Analecta 144. Rome, 1955.
Jeremias, J. Abba: Studien zur neutestamentlichen Theologie und Zeitgeschichte. Göttingen, 1966.
Lefort, L. Les Vies coptes de saint Pachôme et de ses premiers successeurs. Bibliothèque du Muséon 16. Louvain, 1943.
Reitzenstein, R. Historia Monachorum und Historia Lausiaca: Eine Studie zur Geschichte des Mönchtums und der frühchristlichen Begriffe Gnostiker und Pneumatiker. Göttingen, 1916.