A thump is a hard, thudding sound, or an equally hard hit or blow, like the thump your older brother gives you when you embarrass him in front of his friends.
If you drop your backpack full of books it'll make a thump as it hits the floor. And, if your friend starts to choke on his lemonade at lunch, you might want to give him a thump on the back. You can also thump enthusiastically on a drum. Thump is an imitative word, one that mimics the sound of hitting something heavily. It dates from the sixteenth century.