The word 'whistler' is a transparent English compound built from the onomatopoeic verb 'whistle' and the agentive suffix '-er'. The verb 'whistle' itself originates from the Old English hwistlian, a word formed to imitate the high, shrill sound it describes. The suffix '-er', with Proto-Germanic origins, is one of the most productive in English for creating nouns that denote an actor or instrument. Thus, the literal "one who whistles" logically extends from humans and birds to any source of such a sound, and metaphorically to radio signals with a similar tonal quality.

Structure: whistle (to make a high sound) + er (agent suffix) - whistle: From Old English hwistlian (to whistle), of imitative origin. Functional role: The core verb root denoting the action of producing a high-pitched sound by forcing breath through a small opening. - er: From Old English -ere, an agent-noun suffix. Functional role: A derivational suffix that forms a noun indicating a person or thing that performs the action of the verb.