My wife told me this comes from the days when people lived in one-room houses. When mom & dad wanted to get it on, they'd tell the kids to “go buy soy sauce.
Butupsetneverletgrandmaknowvinegarandsoysauce, orI'llhave tolisten tonagging for amonth, longpainrathershort pain, had togo tobuytwobottles of vinegar and a bottle ofsoy sauce.