Someone keeps talking loudly about their new salary and designer clothes, and you whisper your judgment to a friend.

Literally it's like "to fart it," with "péter" meaning "to fart" in slang, which gives it a mocking tone. In practice it means to show off, act arrogant, or try too hard to look important. You use it when someone is bragging, flaunting money/status, or behaving like they're above others. It's pretty common and can be playful between friends or more critical depending on tone.

"Se la péter" does not mean to break something by accident, to apologize sincerely, to practice public speaking. It specifically means "To show off".

Someone reacting to unexpectedly good news at work.

“Gobsmacked” literally suggests being hit in the mouth (“gob” is slang for mouth). In practice it means being stunned into silence by surprise. It’s a vivid and very British way to express disbelief.

"Gobsmacked" does not mean feeling sleepy, laughing uncontrollably, running very fast. It specifically means "Extremely shocked or amazed".