From the Bible, initially Hebrew בן-אדם [Ben-Adam]. Later, it is Bar-Enosh, בר אנש. In the later books in the New Testament it is in Greek [υἱὸς τοῦ ἀνθρώπου].
"When the Beloved One [Christ] becomes transformed into the soul, and when the soul becomes transformed into the Beloved One, that which we call the Son of Man is born from this ineffable, divine, and human mixture. The great Lord of Light, being the Son of the living God, becomes the Son of Man when he transform himself into the Human Soul. The Sun-Man is the result of all our purifications and bitterness. The Sun-Man is divine and human. The Son of Man is the final outcome of the human being. He is the child of our sufferings, the solemn mystery of the transubstantiation." - Samael Aun Weor, The Aquarian Message (1960)
"The Son of Man is born in the Ninth Sphere. The Son of Man is born of Water and Fire. When the Alchemist has completed his work in the mastery of Fire, he receives the Venustic Initiation. The betrothal of the Soul to the Lamb is the greatest festival of the Soul. That great Lord of Light enters Her. He becomes human; She becomes divine. From this divine and human mixture is born that which with so much certainty the Adorable One called “the Son of Man.” The greatest triumph of supreme adoration is the birth of the Son of Man in the manger of the world. The man and woman who love each other are truly two miraculous, harmonious harps, an ecstasy of glory, that which cannot be defined because if it is defined it is disfigured. That is love. The kiss is the profound mystic consecration of two souls who adore each other, and the sexual act is the key with which we become Gods. Gods, there is God. All of you who truly love, know that God is love. To love, how beautiful it is to love. Love is nourished with love; only with love are the alchemical weddings possible. Jesus, the Beloved One, reached the Venustic Initiation in the Jordan. In the moment of the baptism, Christ entered within the adorable Jesus through the pineal gland. The word was made flesh and lived amongst us, and we beheld his glory as the Father’s only son full of grace and truth." - Samael Aun Weor, The Perfect Matrimony (1950)
"In the early parts of the Bible, the original Hebrew for Son of Man is בן-אדם [Ben-Adam]. Later, it is Bar-Enosh, בר אנש. In the later books in the New Testament it is in Greek [υἱὸς τοῦ ἀνθρώπου]. In some cases, the phrase literally refers to a common person. But in others, the meaning is something elevated, superior. In the gospels, the great teacher Jesus refers to the Son of Man often. But he says it in a way that is very interesting. He never says “I.” He talks about the Son of Man in the sense that it is him and it is not him. He says the Son of Man will be persecuted and will rise again. He is talking about himself, but at the same time he is not talking about himself. Why is that? Because he is not the only one. The Son of Man is a symbol of how the force of the cosmic Christ expresses itself through Chokmah, the Second Logos, which is the Son, and becomes humanized in the initiate." - Course: The Book of Revelation