The phrase 'son of man' is a primarily Semitic

Semitic

In linguistics and ethnology, Semitic was first used to refer to a language family of largely Middle Eastern origin, now called the Semitic languages...

 idiom

Idiom

Idiom is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made...

 that originated in Ancient Mesopotamia

Mesopotamia

Mesopotamia is a toponym for the area of the Tigris–Euphrates river system, largely corresponding to modern-day Iraq, northeastern Syria, southeastern Turkey and southwestern Iran.Widely considered to be the cradle of civilization, Bronze Age Mesopotamia included Sumer and the...

, used to denote humanity or self. The phrase is also used in Judaism

Judaism

Judaism ) is the "religion, philosophy, and way of life" of the Jewish people...

 and Christianity

Christianity

Christianity is a monotheistic religion based on the life and teachings of Jesus as presented in canonical gospels and other New Testament writings...

. The phrase used in the Greek, translated as Son of man is ὁ υἱὸς τοὺ ἀνθρώπου. As an idiom for the future human, it can be translated gender-neutrally as offspring of Mankind, or Man's child.
The most common use is similar to that of the English word "human." For example:
1QapGen.

Genesis Apocryphon

The Genesis Apocryphon, originally called the Apocalypse of Lamech and labeled 1QapGen, is one of the original seven Dead Sea Scrolls discovered in Cave 1 near Qumran in the West Bank. Composed in Aramaic, this document consists of four sheets of leather, and is the least well preserved document of...

 XXI.13: MT שיא (Gen.