A lesser known saying that is essentially used when you're emitting an extremely emotional response to something. This could be anger/surprise/humour.
1. Oh wow!!! I just found out my BOYFRIEND is pregnant.. WTF I just shat in a teacup
2. Hidee: Hi mate what are you up to today?
Shawn: Just chillin with my homes
Shawn: Oh did you watch Seinfield the other day, I did find it fairly humerous
Hidee: OMFG YES IT WAS FUNNY. I was watching it and I just did a shit in a teacup!!
3. Peter: oops wrong hole sorry I swear I didnt mean it
Sally: OMFG YOU SICKO. I just did a shit in a teacup