let someone have it attack someone physically or verbally. informal

☞ The interpretation of this idiom was crucial to the outcome of one of the most notorious English murder trials of the 20th century. In late 1952, Derek Bentley and Christopher Craig attempted to rob a warehouse in Croydon. They were trapped on the roof by the police, and Craig pulled out a gun. The police called on him to surrender the weapon. Bentley, who had already been apprehended, shouted out 'Let him have it, Chris!' Did he mean, literally, 'Give the gun to the policeman', or did he mean 'Shoot'? The jury evidently opted for the latter explanation, for Bentley was found guilty of murder and hanged. Craig, who actually fired the shot, was underage and so was only imprisoned.

☞ For other idioms containing let, see the entry for the main word in the idiom (for example, let the cat out of the bag at cat).