Sarcasmes translates to "sarcasm" or "sarcastic remarks". It's a less common word in everyday French songs, making it stand out.
In this song, Serge Gainsbourg uses it to describe the hurtful words directed at him by Marilou, which are the "premières atteintes du mal" (first signs of illness) he experiences. The word perfectly captures the biting and critical nature of the insults, setting the tone for his subsequent physical and mental decline.
Serge Gainsbourg is a master of the miserable, and this song is a perfect example. He describes feeling the first signs of a sickness, brought on by the insults of a woman named Marilou. The words she uses are brutal: "Vieux con, pédale" (Old fool, queer). The effect is physical. He says his ears "Se changèrent en feuille de chou" (turned into cabbage leaves), a direct reference to the cauliflower ear a boxer gets from being hit repeatedly.
Instead of fighting back, he turns on himself. He starts his days buying cigarettes, which he calls "fumigènes" (smoke bombs), and asking the bartender for "le plein de kérosène" (a full tank of kerosene), a fantastic way to describe strong alcohol. The song follows him as he drags himself to a public toilet to vomit up his "alcool et ma haine" (alcohol and my hatred). The transformation is complete when he stumbles out and children laugh at his appearance. He ends with the line, "J'avais pris peu à peu la tronche d'un boxeur" (Little by little, I'd taken on the face of a boxer), beaten down not by fists, but by words.