FOREHEAD. Because of its prominence, its appearance often determines our opinion of the person. In Ezek.16.12 KJV reads “I put a jewel on thy forehead,” but NIV (more correctly) has “I put a ring on your nose.” The forehead is used as a very dishonorable word where in the KJV we read of a “harlot’s forehead” (Jer.3.3; NIV “brazen look of a prostitute”) indicating utter shamelessness. At the same time it stands for courage, as when God told Ezekiel (Ezek.3.9) that he had made the prophet’s forehead harder than flint against the foreheads of the people. The forehead is also the place for the front of a crown or mitre (Exod.28.38), where the emblem of holiness on Aaron’s forehead would make the gifts of the people acceptable before the Lord. A mark was put on the foreheads of the men of Jerusalem who mourned for its wickedness, and they were spared in a time of terrible judgment (Ezek.9.4). Similarly in Rev.7.1-Rev.7.17 God’s servants were sealed by an angel, and it seems that this seal not only saved the elect ones but showed forth their godly character. In the ages of glory that are to come, the name of God will be marked on the foreheads of his own people (Rev.22.4).——ABF
In the NT all the references are in the Book of Revelation. They refer to the fig. or literal marks, seals, or names found on the foreheads of those who are servants of God as well as those who reject God. These marks often show the contrast between these two types of individuals (Rev 7:3; 9:4; 13:16; 14:1; 20:4).
The forehead was also the place where the harlot might advertise her intentions (Jer 3:3; Rev 17:5).
International Standard Bible Encyclopedia (1915)
(2) In a metaphorical sense the expression, "a harlot’s forehead," is used (Jer 3:3) to describe the shameless apostasy and faithlessness of Israel. Eze speaks of the stiff-necked obstinacy and the persistent unwillingness of Israel to hear the message of Yahweh: "All the house of Israel are of a hard forehead and of a stiff heart" (Jer 3:7), and God makes his prophet’s "forehead hard .... as an adamant harder than flint," whereby an unflinching loyalty to God and a complete disregard of opposition is meant (Jer 3:8,9). Compare the phrase: "to harden the face," under the word FACE.