At redMoon's suggestion, I've separated syllables by capitalizing the first letter in each syllable. I've also deviated a tiny bit from standard romanization of Hangul to make pronunciation a little bit clearer.
I've lived in South Korea for about a year and a half, so these should be accurate. Some of the words in the other writeups, I've never seen, or can't recognize because they're strangely romanized, so I can't confirm or deny them. Here are the bad words (some worse than others) I know:
BoJi: Cunt (strongest swear word I know in Korean)
BaBo / Mongchongi / Mangu: Idiot, dumbass
NaPpeun Nyeon: Fucking bitch (very bad)
NaPpeun Nom: Bastard (much milder than Nappeun Nyeon)
ShipPal Nyeon: Fucking slut (also very bad)
MaJjilAe?: Want a beating?
JookEulAe?: Want to die?
MiChyeoSseoh?: Are you crazy?
GoChu: Dick
PanGo: Fart
Mae-Rong!: Nyah-nyah! (a favorite among kids)
KoTakJi: Booger (my friend's favorite line to his students: "KoTakJi MokJiMa!" "Don't eat your boogers!")
OngDong-i: Bum, ass (also the hips)
AkMa GaTeun YeoJa/NamJa: Devilish woman/man
Ddong: Shit
DdongJip: Asshole (as in the part of the anatomy, not as in a bad person; literally means "shit house")
WangTa: Social outcast, loser
YeopGi: Weirdo
Miscellaneous exclamations:
MapSoSa!: Oh my God! (just a generic exclamation of surprise; no literal meaning AFAIK)
SeSang-E!: Similar to Mapsosa!. Literally means "in the world!" so I guess it's similar to the English "What in the world..."
O-TteoGe!: Literally "how!" Used for many purposes, from confusion to excitement.
JinJja?: "Really?" Used as a question, but I've also heard middle school girls using it as an exclamation (combined with playful hitting) directed at their friends when the friend does something embarrassing, like talking to a foreigner. ;-)
A-PpulSsa!: "Oh Hell!" Never heard a Korean actually use this, but they understand what I mean when I use it.
More to be added as I learn/remember them.