'hard-boiled' originates from the literal process of boiling an egg until it becomes firm, which metaphorically extended to describe people who are emotionally tough.

'hard-boiled' was first used in the 19th century to describe eggs and later evolved to describe tough individuals in the early 20th century.

Initially, it meant 'boiled until firm,' but over time it evolved into its current meaning of 'emotionally tough or cynical.'