SOMC, which when expressed in a grammatically correct way would read "Shit on my chest" is a particular expression used to describe frustration. In Australian slang, it is a phrase used on a daily basis to emphasise importance of comment. For example, if one cannot get an electronic device to work properly, one might exclaim "Shit on my chest!", meaning that you are so upset with the device you prefer someone could shit on your chest instead dealing with the situation.
"Dude, I asked her if she knew what SOMCAR was as she was riding me, she said no so I grabbed her sides and started spinnin her around and around. Now she wants to SOMCAR every night!"