Old English hwistlian, from Proto-Germanic *khwis-, of imitative origin. Used also in Middle English of the hissing of serpents. Related: Whistled; whistling. To whistle for (with small prospect of getting) is probably from nautical whistling for a wind. To whistle "Dixie" is from 1940.
"tubular musical instrument," Old English hwistle (see whistle (v.)). 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.
To speculate or take a wild guess