An anti-smile is a facial expression caused by excessive consumption of ecstacy or mdma. The anti-smile will usually manifest itself during the come-down period - the person appears to be smiling upside down.
1. Matey must be off his box, look at that anti-smile.
2. Oi bruv, wat wrong with your mate?
Nothin, hes just anti-smiling
3. How chiseled is ya boy? Look at his anti-smile