butt in

verb2

1. •
   (informal) To join a conversation or situation in which one is not welcome or invited; to interject (connotes a rude or discourteous interruption).

   • Hey, this is none of your business; don’t butt in!
   • Forgive me for butting in.
2. •
   (informal) To join a queue not at the end, but in the middle, in violation of etiquette.