means To produce a clear, high-pitched sound by forcing breath through pursed lips or a small instrument, often as a tune, a signal, or a summons.

from From Old English 'hwistlian,' to whistle — itself an imitative word, the kind language builds by simply mimicking the breathy hiss the mouth makes. It belongs to a small flock of Germanic sound-words (a cousin of Old Norse 'hvísla,' to whisper), all of them little echoes of air slipping through a narrow gap.