Diatribe
Etymology
From Greek "diatribē," "a wearing away" and, by extension, "a pastime" or "a discourse," from "dia," "through," and "tribein," "to rub." Curiously, it first named a serious, unhurried philosophical lecture — time worn away in study. The modern sense reversed the tone entirely, keeping the length but replacing the learning with sustained hostility.
noun · /DYE-uh-tryb/
A long, bitter, and often ranting piece of criticism directed against someone or something.
What it names
The tirade that has slipped its argumentative moorings — where the volume and length signal grievance being vented more than a case being made.
Example
What began as feedback turned into a ten-minute diatribe about everything wrong with the department.
Related idea
A diatribe pours out grievance; an apologia does the structured opposite, mounting a reasoned defense of a position under attack.