The derogatory American-English noun ambulance-chaser designates a lawyer who seeks accident victims as clients and encourages them to sue for damages.

The reference is to lawyers, or their agents, following ambulances taking accident victims to hospital, in order to gain access to those victims.

—Cf. also meanings and origin of the term ‘Philadelphia lawyer’.

The noun ambulance-chaser seems to have originated in New York City. The earliest occurrences that I have found are from Life Topics About Town, published in The Sun (New York City, New York, USA) of Saturday 30^th May 1896 [Vol. 63, No. 273, page 7, column 2]—the formulation “the lawyers known as “ambulance chasers”” seems to indicate that the noun was already in usage:

Anybody who doubts the activity of the lawyers known as “ambulance chasers” would be convinced of their alertness after a short experience in one of the accidents happening every day. Victims of any sort of accident are very promptly deluged with the cards and advertisements of such attorneys; but it used to be necessary for the lawyers to wait until the cases were published in the newspapers. Now such delay rarely occurs. The method of acquainting themselves with such matters has been brought down to a science and offers to obtain legal redress reach the victims of the misfortune rapidly. One instance of especial promptness happened the other day when a man was run over by a wagon in Grand street. This occurred at half past 8 in the morning and before 10 a lawyer had interviewed him and made arrangements to undertake his case. Such rapidity is rather exceptional, but instances little short of it are to be noticed every day. Many of these accidents from driving take place on the crowded streets of the east side and there the news travels quickly to the offices of the lawyers who make a specialty of such practice. They are soon on the hot spot and the victim is readily traced to his home or the hospital to which he may have been carried. The “ambulance chaser” who waits to learn of an accident from the newspapers is regarded now as a very unenterprising lawyer.

The following days, and even weeks and months, newspapers nationwide reprinted, either in part or in its entirety, the article originally published in The Sun (New York City) of 30^th May 1896, thus popularising the noun ambulance-chaser. For example, the article was reprinted in its entirety in The Times (Philadelphia, Pennsylvania, USA) of Sunday 7^th June 1896 [No. 7,560, page 9, column 6], and was quoted as follows in the Camden Daily Courier (Camden, New Jersey, USA) of Tuesday 2^nd June 1896 [Vol. 15, No. 2, page 2, column 1]:

“Ambulance chasers” is the name given to New York lawyers who closely follow up accidents to get damage suits. It is related that a man was run over by a wagon on Grand street the other morning at half-past eight, and before ten o’clock a lawyer had seen him and arranged to bring a suit. Surely, there are no “ambulance chasers” in Camden.

The second-earliest occurrences (including a transferred use) of the noun ambulance-chaser that I have found are from the transcript of a speech that the Republican politician Jonathan Prentiss Dolliver (1858-1910) delivered in Carthage, Missouri, on Monday 5^th July 1897—transcript published in The Carthage Press (Carthage, Missouri, USA) of Thursday 8^th July 1897 [Vol. 26, No. 27, page 4, column 4]:
—This speech “was intended as a direct offset to the recent political effort of Wm. J. Bryan”, i.e., the lawyer and Democratic politician William Jennings Bryan (1860-1925):

“There is in New York a class of lawyers known to the profession as ‘ambulance chasers,’ so called because they are on hand promptly at every railroad accident, street car collision and gasoline explosion offering their advice and services to the wounded in a professional way. I warn my friend Bryan that the business of the ambulance chaser in American politics is speedily coming to an end and that the day is far distant when the American people will deliberately accept a political creed that never has had and never will have any significance except as a distress signal.”

The noun ambulance-chaser then occurred in the following, from The Sun (New York City, New York, USA) of Monday 2^nd August 1897 [Vol. 64, No. 336, page 9, column 2]:

AMBULANCE CHASERS IN ST. LOUIS.
An Occupation the Object of Which Is to Breed Lawsuits.
From the St. Louis Republic.

There are about half a dozen men in this city who make a living in rather a peculiar manner. They have come to be known as “ambulance chasers,” and one or more of them can be found on the scene of almost every accident.
The occupation of the “ambulance chaser” is rather a new one, although the methods he uses have been used to gain the same ends for years back. The “ambulance chaser” is in the employ of some lawyer who makes a specialty of handling damage suits.
When an accident of any sort happens the “ambulance chaser” is right to the front in the crowd which gathers. He gets the name and address of the person who is injured, or if the victim is so badly injured that he cannot give his name and address the lawyer’s agent follows the ambulance to the dispensary, where he usually finds a way to learn what he wishes.
In a few days he calls on the person who was injured and explains to him what a good case he has if he will sue for damages. If the injured person has not sufficient means to prosecute the suit, or shows a disinclination to do so, the “chaser” gets in his fine work and offers to find an attorney who will take the case and carry it through to completion for a percentage of the amount gained as damages. All this at no cost to the plaintiff, for if the suit is lost the lawyer gets no pay. The smooth “chaser” usually succeeds in getting the case on these terms.

The noun ambulance-chasing designates the practice, by a lawyer, of actively seeking accident victims as clients and encouraging them to sue for damages. The earliest occurrence of this noun that I have found is from the following paragraph, published in The Sun (New York City, New York, USA) of Thursday 5^th August 1897 [Vol. 64, No. 339, page 7, column 3]:

“The Sun reprinted the other day from a St. Louis paper an account of the growth there of that branch of legal practice known as ‘ambulance chasing,’” said a New York lawyer. “It may have developed greatly in St. Louis and other towns, but New York’s practitioners of that sharp art would give its followers elsewhere cards and spades and beat them. The matter has indeed assumed such proportions in New York that the Bar Association is about to take action against certain men who are most active in this business, and such influential men will be in the movement that the abuse of law in this form is not likely to continue to flourish as it has for several years past. Liability insurance is rather a new branch of a lawyer’s work, and it has only become settled within the past year or two. The principal opponents of these ambulance chasers will be the large law firms, which look after the business of corporations that are likely to be liable for injuries. These men dealing with the ambulance chasers will be able to accomplish a great deal more than the individual attorneys did who have come into contact with them from time to time, and are familiar with their methods. A great many stories of the activity of these men have been told, of the speed with which they get to people who have been injured, and of the pretexts which they use in order to gain access to patients in hospital. None of these is exaggerated. One or two cases have lately arisen which have showed plainly that the hospital authorities were in league with the lawyers to the extent that they not only allowed them admittance to the patients, but also kept them informed as to new arrivals. This practice has always been suspected, and positive evidence of the fact has recently been discovered. That feature of the business will certainly not be overlooked when the Bar Association begins its campaign against the ambulance chasers. It is very likely to exterminate them, too, for the men back of the scheme are attorneys of high reputation and great influence, who are determined, for the sake of their own profession, that the abuse in this respect shall promptly be brought to an end.”