crowd
駪 centers on a large, moving group: a crowd, especially of horses or people in motion. The given meaning "crowd" captures this idea of many gathered together.
駪 combines 馬 (horse) with 先 (ahead, before), likely suggesting many horses moving forward together, hence a crowd. The exact historical development is uncertain.
Picture a herd of horses (馬) moving ahead (先) in a dense crowd, their hooves thundering together.
For シン, imagine the shin (shin) of a horse as it pushes through a crowd: shin -> シン, and the horse's leg muscles strain amid the throng.