Diss is an abbreviation of the word disrespect. Diss is used as slang, especially among hip hop and graffiti artists. If you diss someone, you have insulted them.
Don't is a contraction of do not. The opposite of don't is do.
Exam is an abbreviation of the word examination. An examination is a test or investigation of something. At school, you may take exams to test your knowledge. A doctor may give you a physical exam to check your health.
A fridge is a short form of the word refrigerator.
Grandpa is short for grandfather.