A phrase used

a) when a situation is so uncomfortable, you'd feel almost less awkward if someone was sucking your butt. Granted, the butt would be the least favorable of places on a human body, any body actually, to suckle.

b) to express extreme distaste or misfortune in a situation.

c) to get people's attention. (With such a profound remark, your hopes, desires, questions and concerns will all be met by this simple phrase.)

When you're being horny (especially around friends) and as a result shame ensues.