A thunder stealer is a very annoying person who bursts in to your conversation and changes the topic to him/herself. Usually leaves you aggravated.
Bob: Hey guys LISTEN! You won't believe what happened today!
Bob: Well i was walking down some alley when i got jumped by three guys with-
Thunder stealer: OH HEY JACK did you do yesterday's homework!?
Jack: Oh yeah, it was pretty easy
Thunder stealer: Hahaha, yeah. Hey can you show me something in the homework? It's on page 54...
Bob: Hey what about my story...