1/. An exclamation made when you notice pairs or multiples of an object or image, ie 2 people wearing the same shirt or driving the same car, or seeing a pair of twins etc. First one to notice the pair loudly says the word 'Snap!' to gain kudos. Originated from an traditional card game where ir a pair is made the word 'Snap!' is shouted, but applied to the real world.
2/. An exclamation used when catching or seeing someone doing something they didnt want anyone to see. Originated from the word 'snapshots' taken by cameras. When catching someone ther word 'snap!' is loudly exclaimed to let them know you saw them. The seen person is then said to have been 'snapped'. Widely used in New Zealand
1./ When I saw she had the same red top and black jeans as that other chick, I had to say "Snap!"
2./ I saw the students smoking behind the pavilion, and they knew it because I shouted "Snap!"