Nerd Seatbelt Share definition Flag 1. (noun) the buckling strap that connects the left- and right-shoulder straps on a backpack. He strode confidently into class, purposefully unbuckling his nerd seatbelt with one hand. Nerd Seatbelt by OUtlET December 29, 2005 0 0 Get merch Get the Nerd Seatbelt mug. Buy Now — $32.95 See more merch