Biblical title of honor, literally "father," used as an invocation of God, from Latin abba, from Greek abba, from Aramaic (Semitic) abba "the father, my father," emphatic state of abh "father." Also a title in the Syriac and Coptic churches.
It is used in the New Testament three times (Mark xiv. 36, Rom. viii. 15, Gal. iv. 6), in each instance accompanied by its translation, "Abba, Father," as an invocation of the Deity, expressing close filial relation. Either through its liturgical use in the Judeo-Christian church or through its employment by the Syriac monks, it has passed into general ecclesiastical language in the modified form of abbat or abbot .... [Century Dictionary]