Uncool means unstylish or not in fashion, like your dad's uncool jeans or the uncool music he likes to blast in the car when he picks you up from school. (How embarrassing!)
The colloquial word cool, meaning "fashionable," or denoting approval in general, has been around since the 1930s and 40s, inspired by jazz slang. Uncool, meaning the exact opposite, first appeared in the 1950s as hipster slang, and it's been in use ever since. So if you're disapproving of or unimpressed with something, it's totally cool to go ahead and call it uncool.