吁 has two distinct readings: yù (籲) means to appeal or call out — used in formal contexts like 呼吁 (to appeal to). As xū, it means a sigh or hush, as in 长吁短叹 — to heave sighs. The traditional form 籲 is used for the yù reading.
Left mouth radical — the indexing element. 吁 began as an interjection of pleading, so the mouth anchor is literal: a sound coming out. Puts 吁 with other voice-noises like 呼 (call out), 叹 (sigh), 喊 (shout), 嘘 (shush). The mouth contributes the speaking-out meaning; the right half supplies the cry.
Right 于 supplies the sound — yú drifting to yù via vowel tightening, the same shift seen in 玗 and 盱. 于 itself depicts a breath of air rising and curving — fitting for a character that names a long, drawn-out exhalation or earnest plea. Same phonetic appears in 宇 (cosmos), 迂 (roundabout), 纡 (twist).
