Vitriolic describes words — writing, speech, criticism, an argument — that are savagely bitter and cruel. Not just angry: angry with an edge that is meant to burn the other person. A vitriolic review does not say 'the film had weaknesses'; it goes after the director personally and enjoys doing it.

It almost always sits in front of a small set of nouns: vitriolic attack, vitriolic criticism, vitriolic language, vitriolic remarks, a vitriolic email, a vitriolic exchange. You would not normally say 'he was vitriolic' about a person's whole character; you say it about what came out of them. It is a strong word, stronger than 'harsh' or 'sharp' — reach for it only when the cruelty is real.

The register is written and journalistic: newspapers, book reviews, reports of political fights and online abuse. In speech people are more likely to say 'brutal', 'nasty', or 'poisonous'. The related noun is 'vitriol', very common in the phrase 'pour vitriol on someone' or 'a campaign full of vitriol'.

One nice thing to know: the word is built on a real chemical. Vitriol was an old name for sulfuric acid, which burns skin. So the picture inside the word is exact — language used as acid.