Short form of testicles. You may also know them as balls, bollocks, cojones, huevos or nuts.
I use all of these terms, and more besides, but then I am Ian Chode, the greatest swearer of modern times!
Chode: Eat my testes, bitch.
Bitch: OK. Mmmmm, better than McDonalds.
2. Exams more difficult than quizzes, but not as difficult as regular tests.
If you think your English professor's 'quizzies' are difficult, then you would hate to see his 'testes'.