'flat-faced' originates from English, specifically the words 'flat' and 'face', where 'flat' meant 'having a level or even surface' and 'face' meant 'the front part or visage'.

'flat' comes from Old Norse/Old English element 'flatr', and 'face' comes via Old French 'face' from Latin 'facies'; the compound 'flat-faced' developed in modern English by joining the adjective 'flat' with the past-participial form 'faced' to describe appearance.

Initially, the components described separate ideas of 'flat' and 'face', but together they evolved to describe a single quality — having a flat or compressed face or front — used for both people/animals and objects.