塞

Bottom earth radical - the indexing radical. Earth is the floor of the container, the foundation that makes the stuffing possible. Also grounds the alternate sài reading (要塞 frontier fortress) - an earthwork blocking a strategic pass. Same family: 堵 block, 填 fill in, 埋 bury.

Top roof radical 宀 - sets the enclosed-space scene. Plugging and stuffing happen inside something: a bottle, a bag, a doorway, a fortress. The roof on top of 塞 marks the bounded interior into which things get crammed. The whole char is indexed under Kangxi #32 土 - the soil at the base.

Middle 7-stroke unit - visually a 工 fused with two pairs of crossing strokes, originally a graph of bricks packed tightly between supports. Standalone 工 is 3 strokes; here it has absorbed the bundling pattern into a single fused unit. The picture: stuff jammed between roof and earth - the 'cramming, stuffing, blocking' core.