Abba
Archimandrite Nikifor’s Biblical Encyclopedia
(Rom. 8:15). This is a Syro-Chaldean word meaning “father”; it occurs only three times in the New Testament, namely: once used by the Lord Himself (Mark 14:36), and twice in the epistles of St. Paul the Apostle (Rom. 8:15; Gal. 4:6). Slaves and bondservants were not permitted to address the head of the household with the word Abba — a circumstance that lends particular force to the words in chapter 8, verse 15, of the Epistle to the Romans. The full meaning of this word cannot be conveyed in our language.
It expresses the highest degree of sincere love, trust, submission, as well as intimate communion. The word Ab (meaning “father”) is used as one of the first and simplest utterances of small children.