Literally: “The Originator of eternity”

The same word is used in Genesis 4:20, 21 – Adah bore Jabal; he was the father of those who dwell in tents and have livestock. His brother’s name was Jubal; he was the father of all those who play the lyre and pipe.

Article by Edward Dalcour – soource: https://christiandefense.org/oneness/isaiah-96-everlasting-father-the-error-of-oneness-theology-refuted/

“For a child will be born to us, a son will be given to us;  and the government will rest on His shoulders; and His name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace. Because of the phrase Eternal Father”  

Oneness advocates argue that the passage is teaching that the prophesied Messiah, Jesus Christ, is the Eternal Father. Aside from the fact, that nowhere in the NT is Jesus ever called “Father,” there are several flaws in this kind of modalistic interpretation:

1. Oneness teachers commit the fallacy of equivocation by asserting that the term “father” has only one meaning. The term father (ab) has various meanings in the OT, depending on the context.
2. When the term father is applied to God (or YHWH) in the OT (only fifteen times), it denotes His parental character to His children, namely, Israel (e.g., Isa. 63:16). Primarily though, the usage of father denoted God as Creator. As a matter of fact, the term “father” is not even a standard recurring title for God in the OT;it is used only fifteen times.
3. The word translated name (shem) as in His name will be called (shem+ qara) was not a formal title for God, but rather it denoted the essence or essential characteristics of who someone is (cf. Young, Commentary on the Book of Isaiah, 1972: 331). This was clearly the Semitic concept of name. Hence, as to the essence and character of the Messiah, He is Wonderful Counselor, Mighty God, Everlasting Father and Prince of Peace.
4. Along with the primary meaning of Creator, the term father correspondingly carries the idea of possessor or founder, as with His creation. For example, 2 Samuel 23:31 speaks of Abialbon, which name means father (or possessor) of strength, strong one. Exodus 6:24 speaks of a man named Abiasaph, whose name means father of gathering, he who gathers. Thus, the Messiah isab of eternity, that is, possessor of eternity. Richards further explains:

 The key word for father in the Bible is ab. It occurs 1,191 times in Hebrew and 9 times in Aramaic form. It is a complex word. Although it usually indicates a literal father or grandfather, it may also be used as a title of respect for a governor or prophet or priest. . . . Ab is also used to indicate the founder of a guild. Thus Ge 4:21 identifies Jubal as father of all who play the harp and flute, i.e., he was the first musician. . . . It is probable that the title Everlasting Father ascribed to Messiah by Isaiah (Isa 9:6) is better understood as father of eternity, i.e., founder of the ages (New International Encyclopedia of Bible Words, 1991: 266).

5. Syntactically, the Hebrew term “father” precedes the word translated eternal (lit. father eternal) indicating the eternal nature of the Messiah. The Aramaic Targums reveal this thought well:

 For us a child is born, to us a son is given . . . and his name will be called the Wonderful Counselor, the Mighty God, existing forever [or He who lives forever ]. The Messiah in whose days peace shall increase upon us (Targum, Jonathan; emphasis added).

6. There has never been a Jewish commentator, Rabbi, or Christian scholar or writer that has interpreted Isaiah 9:6 as Oneness teachers do. Beisner in his book, Jesus Only Churches (1998: 32), dismantles the Oneness exegesis here simply by pointing out that “I am a father, but I am not my father.” Oneness teachers must prove that Jesus is specifically called the “Father” of the Son of God (i.e., His own Father). Isaiah 9:6 only calls Him father of eternity.

“Grace, mercy and peace will be with us, from God the Father and from Jesus Christ, the Son of the Father, in truth and love” (2 John 1:3; emphasis added).