measuring box
桀 is a rare kanji whose core meaning is a measuring box, a container for measuring volume. Its standalone meaning is narrow and concrete.
The origin of 桀 is uncertain. It visually combines 舛 (opposing feet) and 木 (tree), but the historical connection to 'measuring box' is unclear.
Picture a wooden box (木) with a pair of opposing feet (舛) on top, as if someone is stepping on it to measure something. The box is a measuring box.
For ケツ, imagine a measuring box that is so precise it can measure a single kets (invented word for a tiny unit). The sound 'kets' cues ケツ while you picture the box.