Sun radical on the left - the indexing semantic. The square-with-stroke pictograph is the sun. 晚 marks a time-of-day word: the late hours when the sun is low or already gone. Joins the family of solar-time words: 早 (morning), 时 (time), 明 (bright), 暗 (dark), 春 (spring) - all keyed by the sun.
Right component supplies the sound, miǎn → wǎn - initial drift but the rime is preserved. 免 originally pictured a hare (compare 兔). As a phonetic it also yields 挽 (pull), 娩 (give birth) - a wǎn/miǎn cluster sharing the same hare-shape on the right.
