Snot is the discharge that comes from your nose when you have a bad cold. You'll want to carry tissues or a handkerchief with you when you're sneezing, to keep the snot contained.
There are two meanings of the noun snot: the first, mucus, can be unpleasant. The second meaning, an irritatingly arrogant person, is also unpleasant. If a little kid acts like a snot, his babysitter is definitely not going to let him stay up past his bedtime. If you're a snot to your best friend, she might not speak to you the next day. Snot comes from the Old English word gesnot, or "nasal mucus," from a Germanic root.