The word 'swimmer' is a transparent agent noun formation from the Old English verb swimman, meaning "to move through or in water." The verb itself has deep Proto-Germanic roots (swimjaną) and is related to a wider Indo-European base meaning "to be in motion." The suffix '-er', of equally ancient Germanic pedigree, is one of the most productive in English for creating nouns that identify a person by their action or occupation. Thus, the literal and logical construction is perfectly preserved: a "swimmer" is quite simply "one who swims."

Structure: swim (to move through water) + er (agent suffix) - swim: From Old English swimman (to move through water). A Germanic root of Proto-Indo-European origin, functioning as the core verb. - er: From Old English -ere, an agent-noun suffix of Germanic origin, used to form nouns denoting a person or thing that performs the action of the verb.