Green’s Dictionary of Slang

(US) the lip, thus upper/lower decky, the upper or lower lip.

2024

N. Friedman ‘Word of the Week’ 8 July 🌐 a user places a pouch under his top lip (‘pops an upper decky,’ in Zyn argot) or his bottom lip (a lower decky), where it releases (addictive) nicotine into the user’s bloodstream.