Unpleasantness is trouble between people — arguing, bad feeling, rudeness, an ugly scene. It is an uncountable noun, and in this use it names a social event rather than a quality: 'there was some unpleasantness at the meeting' means people quarrelled or someone behaved badly, and everyone felt it. The striking thing about the word is how it is used. It is polite, indirect, a little old-fashioned and very British — a way of pointing at a row without describing it. Speakers reach for it exactly when they do not want to say what happened: 'I'd like to avoid any unpleasantness', 'after all the unpleasantness last year, they don't speak'. That vagueness is the point; the word draws a curtain over the details, and can sound either genuinely tactful or quietly disapproving, sometimes both. Because of this, it is often bigger than it sounds — 'a bit of unpleasantness' can be covering a serious fight. There is also a second, plainer use, closer to the adjective: the quality of being nasty or disagreeable, as in 'the unpleasantness of the smell' or 'the unpleasantness of the task'. That one is not about people at all, just about how bad something is to experience.