Three-drops water radical on the left - the side-form of 水, three abbreviated waves. The indexing radical: 润 originally meant rainwater seeping down and moistening everything below. Groups with 湿 damp, 滋 nourish, 泽 marsh - all watery, soaking ideas.
Right 闰 supplies the sound rùn with no tone shift - a clean phonetic. 闰 pictures a king (王) inside a gate (门), referring to the leap month a ruler proclaimed when the calendar needed correcting. Pure phonetic role here; the calendar meaning doesn't carry into 润.
