A horse, or dialectal/informal term for a large, strong person (often used as a form of address).
Dialectal American English pronunciation/spelling of 'horse'; reflects how regional accents compressed the -r- sound, particularly in Southern and frontier American speech.
Hoss is a perfect example of how dialect becomes spelling—it's just how people actually pronounced 'horse' with a dropped 'r' sound, and it got written down that way.
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