Not cool — but you have to know what 'cool' means first. 'Cool' is a word of praise for a person, a thing, or a way of acting: relaxed, confident, stylish, the kind of thing that makes people want to be near you. 'Uncool' is the opposite of that, and it is a judgement, not a fact. It says: this is embarrassing, this is not what people like us do. Two ways it gets used. About things: uncool clothes, an uncool phone, an uncool band — out of fashion, the sort of thing people quietly laugh at. About behaviour: 'That was really uncool.' Here it does not mean unfashionable at all — it means you acted badly, you were unkind or unfair, you broke the unwritten rule. A friend tells everyone your secret; that is uncool. Notice that this second use is a soft way of saying something hard. It sounds gentle, but the person is genuinely upset with you. The word is informal. It belongs to young people, and it belongs to a certain time — it was strongest from the 1960s to the 1990s, and today it can sound slightly like your parents trying to sound young. Teenagers now more often say 'cringe' or 'that's not it'. Also, the newest slang keeps changing what counts as cool, so 'uncool' quietly means 'uncool to my group' — one group's uncool is another group's normal. A last thing: there is a rare, older use where 'uncool' means not calm, agitated, someone who has lost their cool. You will meet it in old books and songs, not in daily talk.