'baddy' originates from English, formed from the adjective 'bad' plus the suffix '-y' (an informal/diminutive or agentive ending), creating a noun meaning 'a bad person'.

'bad' (Old English root) developed colloquially into 'baddy' in English usage (19th–20th century); later the alternative spelling 'baddie' arose under influence of the -ie diminutive/slang ending, and modern internet slang further broadened the sense.

Initially, it meant simply 'a bad or naughty person'; over time it came to mean specifically a 'villain' in stories and also acquired a separate slang sense of an attractive, confident person in online usage.