This verse sits inside the mother's counsel, Proverbs 31:1–9, and belongs with verse 5.
Quick answer
The second danger: wine in the seat of judgment. The doubled "it is not for kings" is a mother underlining as she speaks; rulers cannot afford clouded heads, for reasons the next verse names.
Read the whole unit
The book's last chapter opens with a unique voice: the words of King Lemuel, "the utterance which his mother taught him." Lemuel appears nowhere else in Scripture, and the counsel he carries is the only teaching in Proverbs explicitly credited to a woman. Its structure is a queen mother's brief: the triple address of maternal urgency (v. 2), then three dangers ranked by what they cost a king: women who consume royal strength (v. 3), wine that clouds royal judgment (vv. 4–5, with a sharp aside on where strong drink might mercifully go instead, vv. 6–7), and finally the positive charge that everything was protecting: open your mouth for the speechless, judge righteously, plead the cause of the poor and needy (vv. 8–9). The logic runs on vocation. A king's strength, clarity, and voice are not his own; they are held in trust for people who have none of the three. The unit ends, and the acrostic begins, and the book closes with the counsel of a mother followed by the praise of a woman, wisdom's last two teachers both unexpected and both unforgettable.
What this verse contributes
This verse contributes the vocational drinking rule, and its logic is office, not abstinence-in-general. Proverbs elsewhere knows wine as both gift and trap; this counsel addresses a man whose decisions determine other people's lives. The doubled negative is the syntax of urgency, and "princes" widens it to everyone who holds judgment over others. What is being protected is not the king's reputation but his faculties: verse 5 will complete the sentence with the people who pay when a ruler's clarity goes. Whoever holds power over the vulnerable, the mother says, has already spent their margin for self-indulgence.
Related: Proverbs 20:1 · Ecclesiastes 10:17 · Isaiah 5:22–23
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.