Left 土 - pictograph of a small mound on the ground line. As the indexing radical it carries the literal meaning: an embankment or dike is a built-up wall of earth holding back water. Same radical groups 堤 with 坝 dam, 城 wall, 墙 wall, 坡 slope, 堆 heap.
Right 是 supplies the sound - shì drifting to dī through an old voiceless-d/sh alternation. Same phonetic frames a tight cluster: 提 to lift, 题 topic, 醍 fine wine, all sharing a tí/dī family of readings. Borrowed purely for pronunciation here.
