口哨
koǔ shào · HSK 7
whistle
😗 口 (mouth) + 哨 (whistle/sentry) = the whistle you make with your mouth
哨 carries the sense of a sentry's whistle-signal; pair it with 口 (mouth) and you get the puckered-lip whistle, not a metal one. 口哨 = the sound, koǔ shào.
口 — pictograph of an open mouth
口 is one of the oldest pictographs in Chinese, originally drawn as a simple outline of an open mouth. Its core meaning 'mouth' extended naturally to 'opening' or 'entrance', as in 入口 (entrance). It also serves as a common radical in characters related to speech, eating, or apertures.