An exam or test which is several orders of magnitude more difficult than expected. Stinkers typically cover content which was otherwise thought to be unimportant and irrelevant and are always significantly harder than any 'practice papers' or questions completed in class. Put simply, stinkers are the bane of all students.
Person 1: Imma get an easy A in maths next week!
Person 2: I wouldn't be so confident, it will probably be a real stinker.