The word 'unsmiling' is a straightforward English formation built by adding the common Germanic negative prefix 'un-' to the present participle 'smiling.' The prefix 'un-' has been a productive way to create opposites in English since its Old English roots. The verb 'smile' itself likely entered Middle English from a Scandinavian source, related to Old Norse 'smjla.' The compound 'unsmiling' logically and transparently describes the absence of the action denoted by its base word, evolving from a simple descriptive phrase into a standard adjective for a serious or stern countenance.

Structure: un (not) + smiling (present participle of 'smile') - un: A prefix of Germanic origin (Old English 'un-'), meaning "not," used to form negative adjectives. - smiling: The present participle of the verb 'smile,' from Middle English 'smilen,' of uncertain origin, possibly from a Scandinavian source.