A more definite form of under than just "under", and further beneath than "beneath" ... therefore, "Underbeneath".
"underbeneath the table..."
The practice of sporting your collar tucked in, typically in crime-riddled urban areas. The polar opposite of the collar pop. Made popular by Atlanta rappers "Tha THC Boyz" (They*Hidin*Collaz.)
Damn, son...You wanna get shot?! You better tuck that collar underbeneath!!!