Green’s Dictionary of Slang

croak v.3

(UK society) to act in a hypocritical manner.

[UK]E. Eden Semi-Attached Couple (1979) 137: Ah, trust you country gentlemen for croaking, and for finding out what is not to be liked; you are never satisfied.
[UK] press cutting in J. Ware Passing Eng. of the Victorian Era (1909) 98/2: John Hollingshead for some time past has been telling his patrons how they croaked in 1807.