1. What does it mean?
뻥 is a casual word for an untrue claim or exaggeration. 뻥이야 can reveal that you were joking, while 뻥 치지 마 tells someone not to lie or make things up.
Also written or said as 뻥치다, ppeong
2. The nuance
It can be playful between friends, but calling someone's account 뻥 challenges their honesty. The same syllable also occurs in sound-symbolic expressions, so not every 뻥 refers to lying.
3. Examples
“하루 만에 책 스무 권 읽었다는 건 뻥이지?”
Reading twenty books in one day is an exaggeration, right?
“나 이사 간대. 아니, 뻥이야. 방만 바꿔.”
I'm moving away. Just kidding, I'm only switching rooms.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.