It's short for "Smoking Dick" and it's a term used for when a chick sucks dick so good that it's as if she's high off of the taste of dick and it's man-juice.
Guy 1: Yo, dis bitch was suckin' my dick so good, I cummed 3 times in less than 30 minutes!
Guy 2: That bitch must've been Smokin' D extra heavy, Goddayum!!!
Guy 2: That bitch must've been Smokin' D extra heavy, Goddayum!!!
by Mr. Ice Cream Man October 25, 2015