Once a Mustang

Alright, listen up, buttercup. This "joke" is about a mustang, which could be a car or a wild horse. The saying means once you're part of the mustang crew (whatever that is), you're always part of it. Mr. Shaw said it, probably because he thinks he's wise, but it's about as funny as a screen door on a submarine. And you? Well, you're probably the type who still uses internet explorer.