the compound "straight-faced" is a relatively modern english formation, combining the adjective "straight," which evolved from the old english concept of being stretched or direct, with "faced," derived from the latin term for appearance or countenance. the literal construction means "having a direct or unaltered facial expression." this imagery logically evolved to describe a person who maintains a serious, impassive, or neutral expression, particularly to conceal amusement or irony, thus connecting the physical description of the face to the abstract concept of emotional concealment.

Structure: straight (direct) + faced (having a face) "straight" from middle english streght, from old english streht (past participle of streccan 'to stretch'), meaning direct or not curved. "faced" from old french face, from vulgar latin facia, from latin facies ('form, appearance, face'), with the english suffix -ed meaning 'having or provided with'.