When something is so peak that the a went flying off. It is a term of increasing popularity amongst the urban youth of London, rivalling that of peak and it often comes accompanied with hand gestures replicating the peak of a mountain.
It can refer to an excellent or top quality situation/object/event/person. (1)
e.g. Pek times for you getting an A* in maths GCSE
Curiously, it can also refer to a very negative situation. (2)
1) Oh man that rave was ^pek^!
2) Dude, your girl is giving you bare shit nowadays, that's pek.