Green’s Dictionary of Slang

beak n.1

[Hotten (1859) + Ware suggest OE beag, a necklace worn as a badge of office]

1. [17C+] (also beeks) a judge, a magistrate; often in phr. up before the beak, in the magistrate’s court [poss. f. harman n.].

2. [19C] (also beaky) a sheriff’s officer, a policeman.

3. [mid-19C] (UK prison/Und., also beakman) a prison warder.

4. [1910s+] a schoolmaster.

In derivatives

beakdom (n.)

[mid-19C] the magistracy.

In compounds

beak-gander (n.) [SE gander, a foolish (old) man]

[late 19C] a senior judge.

beak-runner (n.) [SE runner]

[late 18C] (UK Und.) an officer of the law, lit. ‘a runner for the magistrate’.

beaksman (n.)

[early–mid-19C] a policeman; a police-office clerk.