Old English hwistlian "to whistle," from Proto-Germanic *hwis-, of imitative origin (source also of Old Norse hvisla "to whisper," Danish hvisle "to hiss;" see whisper (v.)). Used also in Middle English of the hissing of serpents; in 17c. it also could mean "whisper." Transitive use from late 15c. Related: Whistled; whistling. At public events, often an expression of support or encouragement in U.S., but often derisive in Britain. To whistle for (with small prospect of getting) is perhaps from nautical whistling for a wind, an old sailor's superstition during a calm. "Such men will not whistle during a storm" [Century Dictionary]. To whistle "Dixie" is from 1940.
"tubular musical instrument sounded by blowing," Old English hwistle (see whistle (v.)). Meaning "sound formed by pursing the lips and blowing" is from mid-15c. To wet one's whistle "take a drink" (late 14c.) originally may have referred to pipes, or be an allusion to the throat as a sort of pipe. Phrase clean as a whistle is recorded from 1878. Railroad whistle-stop (at which trains stop only if the engineer hears a signal from the station) is recorded from 1934.