the word 'whacko' is a modern english slang formation. its first element, 'whack', is onomatopoeic, originating in the 18th century to imitate the sound of a resounding blow. by the early 20th century, 'whack' had developed figurative senses related to mental instability, as in the phrase 'out of whack' or simply meaning 'crazy'. the addition of the colloquial suffix '-o', common in australian and british english to create informal nouns (e.g., 'weirdo', 'kiddo'), solidified 'whacko' as a term for an eccentric or insane person. the term's logic moves from the literal sound of a hit, to the metaphorical idea of being 'hit' or 'out of order' mentally, and finally to a label for the person embodying that state.

Structure: whack (hit) + o (noun/adjective suffix) 'whack' is of imitative origin, likely from the sound of a sharp blow. the suffix '-o' is an informal english suffix used to form nouns and adjectives, often with a familiar, humorous, or derogatory sense.