Green’s Dictionary of Slang

rook n.1

[the allegedly larcenous character of the bird]

1. [late 16C+] (UK Und.) a cheat or swindler.

2. [late 18C–early 19C] (UK Und.) a small crow bar [pun; a reverse of nature, where the rook is larger than the crow].

3. [mid-19C] a clergyman [the black clothes or, according to Hotten (1864) f. the nursery rhyme ‘Who Killed Cock Robin?’, ‘I, says the Rook, / With my little book, / I’ll be the parson’.].

4. [1930s+] a swindle.