The Rice University Neologisms Database

jewelry worn across the front teeth Grill has been used longer to refer to the mouth, especially the grill-like facade formed by smiling teeth. Through metonymy, this simply attached itself to the adornment which was placed across the teeth. There are certainly earlier uses of this term than 2005, at least among the Houston rap community, but I couldn’t specifically cite them, and I had not heard the word used to mean jewelry until this year.