Fausset's Bible Dictionary
The Chaldaic-Hebrew form, as ab is the Hebrew form, for the Greek pater , "father." Instead of the definite article which the Hebrew uses before the word, the Chaldee or Aramaic adds a syllable to the end, producing thus the emphatic or definitive form. It is used to express a vocative case, and therefore is found in all the passags in which it occurs in the New Testament (being in all, an invocation): Mark 14:36; Romans 8:15; Galatians 4:6.
The use of the Hebrew and of the Greek appellation addressed to the one Father beautifully suggests that the Spirit of adoption from Jesus, who first used the double invocation, inspires in both Jew and Gentile alike the experimental knowledge of God as our Father, because He is Father of Jesus with whom faith makes us one, and as our God because He is Jesus' God. Compare John 20:17, "ascend unto My Father and (therefore) your Father. and to My God and (therefore) your God"; Galatians 3:28, "there is neither Jew nor Greek, for ye are all one in Jesus Christ"; Ephesians 2:18, "through Him we both have access by one Spirit unto the leather." (Especially (See above.) "Abba" was a title not to be used by slaves to a master, nor Imma to a mistress, only by children: see Isaiah 8:4. "Before the child shall have knowledge to cry Abi, Immi.")
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Fausset, Andrew R. Entry for 'Abba'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/abba.html. 1949.