The Brief: SMH means "shaking my head."
SMH conveys disapproval or judgment; it can even be used to throw shade. Often, in an effort to underscore the feeling of disdain, GIFs of actual head shaking are included with the acronym. Variations of SMH include SMDH (shaking my damn head) and SMFH (shaking my f**king head).
It’s always “wyd” it’s never “Do you want some crab legs?” smh
— Jp 🤤💦 (@jadynelysiaa) January 28, 2019