Old English hwisprian "speak very softly, murmur" (only in a Northumbrian gloss for Latin murmurare), from Proto-Germanic *hwis- (source also of Middle Dutch wispelen, Old High German hwispalon, German wispeln, wispern, Old Norse hviskra "to whisper"), from PIE *kwei- "to hiss, whistle," imitative. Transitive sense is from 1560s. Related: Whispered; whispering. An alternative verb, now obsolete, was whister (late 14c., from Old English hwæstrian), and Middle English had whistringe grucchere "a slanderer."

1840, from steam (n.) + whistle (n.).

also whistle-blower, 1963 in the figurative sense, American English, from whistle (n.) as something sounded in an alert + agent noun from blow (v.1).

Old English hwistlere "piper," literally "whistler," agent noun from hwistlian (see whistle (v.)).

Old English hwistlung, verbal noun from hwistlian (see whistle (v.)).