I'd like to know the meaning of "truculent" and if the phrase "What's all that?" in the following excerpt from chapter Seven of The Great Gatsby is an idiomatic expression meaning "what did he say to you"?
Only the Negro and I were near enough to hear what he said but the policeman caught something in the tone and looked over with truculent eyes.
“What’s all that?” he demanded.
“I’m a friend of his.” Tom turned his head but kept his hands firm on Wilson’s body. “He says he knows the car that did it... It was a yellow car.”
Fitzgerald is characterizing a New York cop of the 1920s. He is not overly intelligent or educated. He is probably Irish. He might even be an Irish immigrant. He is feeling confused and frustrated because he is trying to handle a complicated situation all by himself, and there are a lot of people present, a mixed assortment of relatives, friends, neighbors, and onlookers. The word "truculent" suggests fierceness and belligerence, but this is only because the cop is trying to assert his authority and control a bunch of people, many of whom are virtually ignoring his existence. He is ready to use violence if he feels the situation is getting out of his control. He may be acting fierce, but he is really feeling anxious, alarmed, nervous, and ill at ease because he has never been involved in a death before and doesn't really know what he should be doing.
The expression "What's all that?" might be described as idiomatic, but it is possibly more Irish than American--something the cop brought with him from Ireland or picked up from Irish immigrants. It is something that a New York Irish cop of the 1920s would be likely to say when trying to assert his authority but not something that would be common in the American vernacular. The cop only means, "What's all that talking about?" In Ireland they might say, "What's all that blather?" or "What's all that fuss about?" "What's all that?" was never a popular American idiom.
Fitzgerald's first mention of the cop is suggestive.
Myrtle Wilson's body, wrapped in a blanket, and then in another blanket, as though she suffered from a chill in the hot night, lay on a work-table by the wall, and Tom, with his back to us, was bending over it, motionless. Next to him stood a motorcycle policeman taking down names with much sweat and correction in his little book.
This cop is not even a patrolman but a motorcycle policeman. His job is to hand out tickets for traffic violations. He was forced to intervene in this situation because he is the only policeman in the vicinity and because a car was involved. All he can think of doing is to take down names and act as if he is in charge of the situation. In those days he would have had no means of communicating with his station except via telephone. If he asks to use the Wilson's phone he will lose what little control he has over the crowd, but he would dearly love to be able to turn the investigation over to a superior or at least ask for instructions. His asking, "What's all that?" shows that he is getting more input than he is able to digest and process. He certainly doesn't like Tom barging in and interfering with his case by talking to the victim's husband in such an urgent tone. He doesn't catch what Tom is saying, but Tom's tone of voice makes him suspicious. So far, the motorcycle policeman doesn't understand what happened and doesn't know the identity of a single person including the victim.