'single-faced' originates from a combination of the adjective 'single' and the noun 'face'. 'Single' ultimately comes from Latin 'singulus' (via Old French 'sengle'/'single'), and 'face' comes from Latin 'facies' (via Old French 'face').

'single' entered Middle English from Old French 'sengle' (from Latin 'singulus'); 'face' entered English from Old French 'face' (from Latin 'facies'). The compound 'single-faced' is a Modern English formation combining these elements to describe something with one face or, by extension, someone not 'two-faced.'

Initially the components meant 'one' ('single') and 'face' ('face'), giving a literal sense of 'having one face.' Over time the compound has also been used figuratively to mean 'not two-faced' or 'sincere.'