Jer 17:9 (LXX) reads:
The heart is deeper than all things; a person is also. And who will understand him? (Lexham English Bible)
Interestingly, early Christian authors used Jer 17:9 (LXX) to apply to none other to the person of Jesus:
“[I]s it He who is from the Virgin, Emmanuel, who did eat butter and honey, of whom the prophet declared, ‘He is also a man, and who shall know him?’” - Irenaeus, Against Heresies III, 18
“For this reason [it is said], ‘Who shall declare His generation?’ since ‘He is a man, and who shall recognize him?’ But He to whom the Father which is in heaven has revealed Him, knows Him, so that understands that He who ‘was not born either by the will of the flesh, or of the will of man, is the Son of man, Christ, the Son of the living God.’” - Irenaeus, Against Heresies III, 19
“And whatever other things of a like nature are spoken regarding Him, these indicated that beauty and splendour which exist in His kingdom, along with the transcendent and pre-eminent exaltation [belonging] to all who are under His sway, that those who hear might desire to be found there, doing such things as are pleasing to God. Again, there are those who say, ‘He is a man, and who shall know him?’ and, ‘I came unto the prophetess, and she bare a son, and His name is called Wonderful, Counsellor, the Mighty God;’ and those [of them] who proclaimed Him as Immanuel, [born] of the Virgin, exhibited the union of the Word of God with His own workmanship, [declaring] that the Word should become flesh, and the Son of God the Son of man (the pure One opening purely that pure womb which regenerates men unto God, and which He Himself made pure); and having become this which we also are, He [nevertheless] is the Mighty God, and possesses a
generation which cannot be declared.” - Irenaeus, Against Heresies IV, 33
“And then shall they ‘learn to know Him whom they pierced, and shall beat their breasts tribe by tribe;’ of course because in days bygone they did not know Him when conditioned in the humility of human estate. Jeremiah says: ‘He is a human being, and who will learn to know Him?’ because, ‘His nativity,’ says Isaiah, ‘who shall declare?’ So, too, in Zechariah, in His own person, nay, in the very mystery of His name withal, the most true Priest of the Father, His own Christ, is delineated in a twofold garb with reference to the two advents.” - Tertullian, An Answer to the Jews, 14
“For it is of Him that Isaiah writes: ‘A man of suffering, and acquainted with the bearing of weakness;’ and Jeremiah: ‘He is a man, and who hath known Him?’ and Daniel: ‘Upon the clouds (He came) as the Son of man.’ The Apostle Paul likewise says: ‘The man Christ Jesus is the one Mediator between God and man.’” - Tertullian, On the Flesh of Christ, 15
“For Jeremiah likewise, he says, was aware of the Perfect Man, of him that is born again—of water and the Spirit not carnal. At least Jeremiah himself remarked: ‘He is a man, and who shall know him?’ In this manner, (the Naassene) says, the knowledge of the Perfect Man is exceedingly profound, and difficult of comprehension.” - Hippolytus, Refutation of All Heresies, 3
“In Jeremiah: ‘And He is man, and who shall know Him?’ Also in Numbers: ‘A Star shall arise out of Jacob, and a man shall rise up from Israel.’ Also in the same place: ‘A Man shall go forth out of his seed,’ and shall rule over many nations; and His kingdom shall be exalted as Gog,’ and His kingdom shall be increased; and God brought Him forth out of Egypt.” - Cyprian, Against the Jews, II
“But that the Jews, by whom He behoved even to be slain, were not going to acknowledge Him, this prophet thus intimates: ‘Heavy is the heart through all; and He is a man, and who shall know Him?’. That passage also is his which I have quoted in the seventeenth book concerning the new testament, of which Christ is the Mediator.” - Augustine, City of God, 33
“The prophet explains this in the words in which he seems to check himself, and to supply the omission: ‘His heart,’ he says ‘is sorrowful throughout; and He is man, and who shall know Him?’ He is man, in order that in the form of a servant He might heal the hard in heart, and that they might acknowledge as God Him who became man for their sakes, that their trust might be not in man, but in God-man.” - Augustine, Reply to Faustus the Manichæan, 8
(my thanks to my friends Christopher Davis and Errol Amey)