A man never reaches that dizzy height of wisdom that he can no longer be led by the nose.
To lead by the nose means that you make someone do something they would otherwise be reluctant to do by faking good intentions or outright lying to them about their reward.
The phrase comes from very ancient times, when farmers and ranchers would lead their horses and other animals literally by the nose - placing a ring through the beast's nostrils for a leash - and often to places the animals didn't want to go, such as the butcher shop. In time, the phrase came to mean anyone being led reluctantly, usually to their doom.
The Bible makes a passing mention of the metaphorical usage of the phrase, in Isaiah 37:29:
Because thy rage against me, and thy tumult, is come up to mine ears, therefore I will put my hook in thy nose, and my bridle in thy lips, and I will turn thee back by the way by which thou camest.
The phrase "led by the nose" makes its first appearance in William Shakespeare's Othello, Act I, Scene 3, with Iago revealing to the audience his plans to ruin Othello by telling the king that his wife is having an affair with Cassio:
After some time, to abuse Othello's ear
That he is too familiar with his wife.
He hath a person and a smooth dispose
To be suspected, framed to make women false.
The Moor is of a free and open nature,
That thinks men honest that but seem to be so,
And will as tenderly be led by the nose
As asses are.
Although the phrase often time implies that the person being led is gullible and foolish, but there is another connotation: that the person doing the leading has no idea what direction to head, but still feels compelled to lead people in that direction. When people are complaining of being led by the nose, they are often complaining that, in addition to being lied to by their leaders, the leaders don't know the right direction to take, and are incompetent in addition to untrustworthy.