This title is first applied to Jack Salmon by the police in Chapter Eleven, which charts the way that the police, and in particular Len Fenerman, become increasingly concerned about the way in which Jack Salmon becomes so fixated on the guilt of his neighbour, Mr. Harvey, in relation to his daughter's disappearance. Note what the "final straw" is in relation to this matter:
The final straw had been a call that came in the first week of July. Jack Salmon had detailed to the operator how, on a morning walk, his dog had stopped in front of Mr. Harvey's house and started howling. No matter what Salmon had done, went the story, the dog wouldn't budge from the spot and wouldn't stop howling.
It is this kind of behaviour from Jack Salmon that moves the feelings of the police from sympathy, kindness and understanding towards annoyance and humour. Thus the name that is given to him reflects the way in which Jack Salmon uses whatever evidence he can to support his hunch that Mr. Harvey killed his daughter without any identifiable proof.