Someone who ignores the unspoken "one and one and one and one" rule in traffic when two lanes merge together.
When this "zipper" rule is not adheared to, it causes a snag in traffic.
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.
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!