'plain-faced' is a compound formed in Modern English from 'plain' + 'faced'. 'Plain' originates from Old French 'plain' (from Latin 'planus') meaning 'flat, level' and 'face' originates from Old French 'face' (from Latin 'facies') meaning 'appearance' or 'face'.

'plain' entered Middle English from Old French 'plain' (ultimately Latin 'planus'), and 'face' came via Old French from Latin 'facies'. These elements were combined in English to form compound adjectives such as 'plain-faced' to describe appearance; later usage extended figuratively (e.g., 'plain-faced lie').

Originally the components referred to a 'flat/level appearance' or simply an unadorned face; over time the compound came to mean both 'plain in appearance' and, by figurative extension, 'openly undisguised' (as in a blatant lie).