What this stands for depends on whether or not you're 13 years old, or not.
If you're a hobbyist (like myself) who likes working with printed circuit boards (PCBs), SMD stands for Surface Mount Device, which are devices that are soldered onto the board from only one side, in contrast to THT (Through
Hole Technology) devices.
And if you're a prepubescent
13 year old, I suppose SMD stands for "suck my dick", just like everyone else who has defined this term thus far on here...