[dǎ jiàng yóu]
Literally "buy soy sauce".
1. (Kid) mature enough to run errands or do household chores.
She got married at 20; her son is already a preteen.
2. None of my business. This expression originates from an Internet meme: when interviewed by a local TV station about Edison Chen's sex photo scandal in 2008, an unnamed Guangzhou resident answered, "关我鸟事。我是出来打酱油的。" (It's none of my freaking business. I come over here to buy some soy sauce.)
I'm not with the gang. I'm just a passerby.
A derivative meaning is "doing nonessential work / being an assistance or follower" commonly used in self-deprecating jokes.