"Son of man" is a frequent phrase on the OT, especially Ezekiel, (eg, 2:6, 11:15, 12:27, 13:2, 20:3, 21:19, 22;18, 24:2, 28:2, 30:21, 32:18, 24, 33:2, 12, 20, 34:2, 36:1, 37:16, 38:2, 39:17, 40:4, 43:7, 45:5, etc.) See also Job 25:6, Ps 8:4, 80:17, 144:3, Isa 56:2, etc. The phrase also occurs in the plural, "sons of men".
In all these, it simply means, "a mortal man/human" or similar.
The obvious exception to this is the occurrence in Dan 7:13 because it is alluded to in the NT:
Dan 7:13 - “I kept looking in the night visions, And behold, with the
clouds of heaven One like a son of man was coming, And He came up
to the Ancient of Days And was presented before Him.
Commenting on this passage, Ellicott observes:
(13) The Son of man.—Hence our Saviour adopts the title which
designates Him as Judge (Matthew 24:27, &c.). The title implies one
descended from man; but as this Person is spoken of as being “like”
one of human descent, it follows that He was not merely a man. The
early Jewish and Christian interpretations that this is the Messiah
are confirmed by our Saviour’s solemn appropriation of the title to
Himself (Matthew 24:30). In this verse the judgment is supposed to
have already taken place upon earth, and the Son of man comes in the
clouds to claim His kingdom.
The Cambridge Commentary is also useful:
with the clouds of heaven in superhuman majesty and state. The passage is the source of the expression in Mark 14:62 (Matthew 26:64
‘on’); Revelation 1:7, ‘behold, he cometh with the clouds:’ cf.
Matthew 24:30 (‘on’) = Mark 13:26 (‘in’) = Luke 21:27 (‘in’); and
Revelation 14:14 (‘one sitting on a cloud, like unto a son of man’),
Significantly, Jesus is the only person qualified to take both titles:
- "Son of Man" because He was biologically descended from humans because He was the Son of Mary
- "Son God" because He was also (biologically??) descended from God as per Luke 1:35 which encapsulates both -
The angel replied, “The Holy Spirit will come upon you, and the power
of the Most High will overshadow you. So the Holy One to be born will
be called the Son of God.
Jews well versed in Scripture would have been familiar with the Messianic prophecy in Dan 7:13 which also provided another source of wonder for them - How could this humble carpenter be Messiah and King of Israel, the "Son of Man" prophesied by Daniel?? Little wonder they wanted to kill Him.
Son of Man in the NT
In the NT, the title for Jesus, "Son of Man" occurs 86 times, 82 of these in the Gospels such as Matt 8:20, 9:6, 10:23, 11:19, 12:8, 32, 40, etc, and almost always in the mouth of Jesus. Indeed, it was (based on frequency) Jesus favorite title for Himself - in the Gospels the title is used only in the mouth of Jesus.