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.
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Definitions of whistling from WordNet
the sound made by something moving rapidly or by steam coming out of a small aperture;