Rare and somewhat literary term for a person one knows; modern Japanese normally uses 知人 (ちじん) or 知り合い (しりあい).
The old letter used the word 'shiribito' (acquaintance).
Compound of 知り (shiri, continuative form of 知る 'to know') and 人 (hito, 'person'), with rendaku voicing to びと. The exact historical development is uncertain, but the form follows a classical pattern for nominal compounds.