Acts 23:21
But do not thou yield unto them: for there lie in wait for him of them more than forty men, which have bound themselves with an oath, that they will neither eat nor drink till they have killed him: and now are they ready, looking for a promise from thee.
More than forty men had bound themselves with an oath to eat nothing and drink nothing until they had killed Paul. They were then ready, waiting for the Roman commander to promise to bring Paul before the council. The oath and the request to produce Paul were part of a planned ambush to kill him.
The plot was murder disguised as religious zeal, with the conspirators using a solemn oath and fasting to support their plan. The priests and rulers agreed to the conspiracy. Paul's nephew learned of the plot and warned Paul about it. The young man urged the commander not to yield to the conspirators and not to grant their request.