I'm not sure why it took me so long to figure this out, but the last people you should take Korean language advice from are kyopos (foreign-born or raised Koreans). That being said, if you do follow their advice, you will get many laughs from Koreans. Some of my personal favorites, all of which actually happened to me:
- When I first got to Korea, I was at some open-air event, and during a break I started talking to one of the hosts. He said he was only a part-time host, so I asked him what his full-time job was, and he said "백수" (which is slang for "unemployed guy"). I asked him what that was, and he replied, "Comedian". So then the next few people I met, I proudly told I was a baeksu. (Edit: Actually, this guy was Korean Korean, not kyopo.)
- Next, a kyopo who lived in the apartment I moved into back in 2010 asked me what I was doing in Korea, and I told him I was starting a company, and asked how to say that in Korean in case people ask. He told me the way to say "starting a company" is "싸우다", which literally means "to fight", but apparently is a slang way of saying "to do business". I bought it, and the next day on the subway, a random stranger asked me what I was doing in Korea, and sure enough, I said, "싸우다". Thinking they misheard, they asked again, and again I replied that I was in Korea to fight. They quickly moved away.
- My favorite might be a recent occurrence. While eating "흑돼지", my kyopo friend asked me if I knew what "흑" meant, and I said, "Yeah, it means 'black', right?" No no, came the reply. "흑" means "dirt", so it's just slang for "black". Knowing that "흑인" is what Koreans call black people, I was like, "Uh, does that mean all this time Koreans have been calling black people "dirt people"? That's fucked up!" Kyopo friend says, "Yeah, isn't that fucked up?"
Yes! That is fucked up! Especially if it were true, which it isn't. "흑" means "black", so "흑인" literally means "black person". The word for "dirt" is "흙", which in most parts of Korea is pronounced exactly the same as "흑". Korea might be a racist country, but in this instance the reality is far more benign than the kyopo imagination.
Lesson learned - whatever you do, don't learn Korean from kyopos!