Back in the 1950s and '60s there were ads in the back pages of comic books for the Charles Atlas body building programs.
Charles Atlas - Wikipedia, the free encyclopedia These ads were drawn in the style of comic book panels and showed a "97 pound (44kg) weakling" at the beach being teased by a muscular bully who kicks sand in the face of the weakling in front of an attractive girl. The weakling then sends away for the body building kit, follows the instructions, becomes muscular, goes back to the beach, beats up the bully, and is admired by the attractive girl.
Having sand kicked in your face means to be insulted by someone or something you are powerless to fight back against.