'ballsy' originates from colloquial English, specifically from the word 'balls' (slang for testicles) combined with the adjectival suffix '-y', where '-y' meant 'characterized by' or 'full of'.

'ballsy' developed in 20th-century English as a slang adjective formed from the figurative use of 'balls' to mean 'courage' (earlier usage of 'balls' in the sense of nerve appears in 19th/early 20th century); comparative and superlative forms like 'ballsier' and 'ballsiest' followed regular adjective inflection.

Initially literal references to 'testicles', the figurative sense of 'balls' as 'courage' produced 'ballsy' meaning 'courageous' or 'audacious'; over time it has remained in that informal/vulgar register and broadened to describe bold or risky behavior.