an expression from the sixties and seventies used to describe what is known as today, "epic fail".
Usually if someone does incredibly stupid it can be used as an interjection and when you say, "snag" you must make an O.K. sign with your fingers and hold it out, away from your body, rather than up.
The "a" is pronounced long.
Peter: Haha, what happened to your shirt?
Bill: i was in the park today and i was eating an ice pop, when it fell off the stick and onto my shirt.
Peter: Snag, man!