half-shitted grin
A grin for one of those moments where you say or do something that is so unbelievably dumb, the only thing you can do is grin in a ridiculous manner. Bear in mind, the said action is literally so fucking stupid, no normal grin can convey the amount of stupidity your brain just expressed. (i.e. The face you make when your brain has a severe case of the shits.)
Sam walked into her kitchen and saw her boyfriend butchering yet another cow on the table they ate dinner from. When he noticed the angry look she gave him, all he was capable of was a half-shitted grin. This was the third time this month.