'mate' (plural 'mates') originates from Old English/Middle English roots meaning 'one met' or 'companion', ultimately from a Proto-Germanic source related to meeting or partnership.

The word appeared in Middle English as 'mate' meaning companion; it developed from Old English elements related to 'meeting' and was reinforced by cognates in other Germanic languages, eventually becoming the modern English 'mate' and its plural 'mates'.

Initially, it referred simply to someone 'met' or encountered (a companion or partner); over time it kept the sense of companion and broadened into informal use for 'friend' and specialized senses (e.g., ship's mate, sexual pairing).