Korean ↔ English slang, explained by the people who actually use it.

It is an abbreviation of "suddenly pops out."

"Mom's friend's son" — used for someone impossibly accomplished compared to you. The female form is "eomchindal" (mom's friend's daughter); there's also "bu-chinnam" (wife's friend's husband).

Originally from a polite gamer sign-off ("enjoy the game"), it got shortened over time and repurposed to mean "forget it" or "get lost" — often used to shut down an absurd item-trade offer or dismiss someone outright.