Thayer's Greek Lexicon

 Abba = "father"
1. father, customary title used of God in prayer. Whenever it occurs
in the New Testament it has the Greek interpretation joined to it,
that is apparently to be explained by the fact that the Chaldee
"ABBA" through frequent use in prayer, gradually acquired the
nature of a most sacred proper name, to which the Greek speaking
Jews added the name from their own tongue.