The original title of this 1973 French film is "L'emmerdeur, which translates as "the bother" or "troublemaker." When it made it to the States in 1975, neither of those translations must not have had the box office appeal, so it got the more attention-grabbing title, "A Pain in the Ass."
By whatever name, this French film is a very good dark comedy about a hit-man whose contract killing is constantly foiled by a fumbling salesman. The salesman himself botches an attempt at suicide over his wife leaving him for her analyst. It's worth noting for modern audiences that from about the mid-1960s through the 1970s, it was a big thing for people to have an analyst or psychiatrist. At least, that's what many comedy films portrayed about the wealthy and average skyscraper workers of the Big Apple. This was almost always done with light satire or outright mockery.
The plot, settings and characters in this film with English subtitles are very good. MGM in 1981 remade the film, changing the setting to California and enough of the rest of the plot and characters that it tanked. "Buddy Buddy" starred Jack Lemmon and Walter Matthau, but one could never quite seriously see Matthau as a hit man, where Lino Ventura was convincing in the French original. Lemmon's Victor Clooney came across more as a whining wimp, where Jacques Brel is very good as a good-natured, if somewhat scatter-brained salesman who wants to get his wife back.
Then, the American version had a totally different psychiatric aspect with Klaus Kinski playing Dr Hugo Zuckerbrot who ran a desert sex farm. That totally did in the MGM production. In this film, Jean-Pierre Darras plays Dr. Fuchs who has a bona fide psychiatric institution, which serves to fuel the plot and the antagonists nicely.
The later American film also removed the dark edge, which may have further dimmed any audience consideration of Matthau's character as a hit-man. This original film has early scenes with a car-bomb explosion and then a failed first hit-man being taken out by Ventura's Ralf Milan, who shoots him in the head.
This is a very good and enjoyable comedy. It's not for the whole family, but those who aren't squeamish about a little blood and rough stuff as part of comedy should find this film anything but a pain in the ass. That would have been a much better title for the Lemmon-Matthau remake of 1981.