'straight-faced' originates from the combination of 'straight' and 'face', where 'straight' meant 'direct' and 'face' referred to 'facial expression'.

'straight-faced' evolved from the literal meaning of having a 'straight' or 'serious' face, eventually becoming an idiomatic expression for maintaining a serious demeanor.

Initially, it meant 'having a direct or serious facial expression', but over time it evolved into its current meaning of 'hiding amusement or irony with a serious face'.