'bonkers' originates from English slang, probably influenced by the verb 'bonk' (to strike) or the dialectal 'bunk' (nonsense), with the adjectival/slang suffix -ers added; earliest recorded uses are 20th-century British informal speech.

'bonkers' appears in early-to-mid 20th century British slang; it likely developed from the verb 'bonk' (onomatopoeic 'to strike' or 'hit the head') or was influenced by 'bunk' meaning 'nonsense,' and then took the form 'bonkers' as an adjective describing someone mentally affected.

Initially associated with the idea of being 'struck' (literally or figuratively) or with 'nonsense,' it evolved into the modern informal sense of 'crazy' or 'very enthusiastic/obsessed' depending on context.