1. actual word in Korean for older boy/big brother-type that idiot white kids have turned into a racial slur.
2. sign you're talking to a white kid who thinks appropriating Asian culture is cool and not offensive cause they're into K-Pop and J-Pop. Often paired with an adjective ("one sexy oppa" = hot guy, "curdled milk oppa" = ugly guy, etc.)
"Kim Hyung Joong is my favorite oppa!" said the white kid who rapes Asian culture.