Something that's unsuitable doesn't work well or is out of place. A kitchen knife is unsuitable for sawing firewood.
You can use the adjective unsuitable to describe something that isn't well suited for a use: flip flops are unsuitable shoes to wear in a blizzard. It's also a good word for something that's disappointing or undesirable: "No, that guy in the clown suit would be a completely unsuitable boyfriend for me." At the root of unsuitable is the meaning of suit, "be agreeable or convenient," that's thought to come from "provide a suit of clothes."