Is it farther or further?
Old English hwisprian "speak very softly" (only in a Northumbrian gloss for Latin murmurare), from Proto-Germanic *khwis- (cf. Middle Dutch wispelen, Old High German hwispalon, German wispeln, wispern, Old Norse hviskra "to whisper"), imitative and probably related to Old English hwistlian "to whistle." Related: Whispered; whispering. The noun is from 1590s.
speech in which the vocal cords are held rigid, preventing the vibration that produces normal sounds. In whispering, voiceless sounds are produced as usual; but voiced sounds (e.g., vowels) are produced by forcing air through a narrow glottal opening formed by holding the vocal cords rigid and close together. See also voice; vocal fry.