A threesome. Sex involving three people - two of which are female. A jaffle is an Australian term for a sealed toasted sandwich. To be sandwiched between two fannies.
When you get a hand job while your dick is inside your underwear and the hand is on the outside. By the time you blow your load, it's hot and crisp on the outside from the friction and warm and moist on the inside. Making a perfect Jizz Jaffle.