Green’s Dictionary of Slang

(UK Und.) ironic description of the gasps of alleged ‘surprise’ from the accused when the police produce their evidence in court.

1977		D. Powis Signs of Crime 181: Dock asthma Pretended gasps of surprise by accused persons when incriminating evidence is given against them. Used generally to describe the reaction to any unpleasant surprise, whether in court or not.
2002		P. Bean Drugs and Crime cited in Guardian Rev. 21 Dec. 🌐 I was aware that the chairman was stricken with a lively attack of what is known in the trade as ‘dock asthma’: spluttering, shock, amazement, and a ‘what’s he talking about?’ expression.