What is the meaning of "water and blood" in 1 John 5:6?

In 1 John 5:6 John uses water and blood as symbolic short hand, clearly expecting his audience to know the meaning of these two liquids.

6 This is the one who came by water and blood—Jesus Christ. He did not
come by water only, but by water and blood. And it is the Spirit who
testifies, because the Spirit is the truth. 7 For there are three that
testify: 8 the Spirit, the water and the blood; and the three are
in agreement. 9 We accept human testimony, but God’s testimony is
greater because it is the testimony of God, which he has given about
his Son.

At its most basic 1 John 5:6 operates as a counter claim to those who hold that Jesus Christ came in water but not in blood. John holds that Jesus Christ came by both water and blood. A counter group appears to believe He came only in water. The structure of this verse as an implicit rebuttal begs the question of what does it refute. What is the meaning of water and blood?