'arbitrationist' originates from English, specifically the noun 'arbitration' + the suffix '-ist', where 'arbitration' meant 'settlement by an arbitrator' and '-ist' meant 'one who practices or supports (a doctrine or activity)'.

'arbitration' comes from Latin 'arbitrationem' (from 'arbiter' meaning 'judge' or 'decider'), passed into Old French and then Middle English as 'arbitracioun'/'arbitration'; the agentive suffix '-ist' entered English via Latin/Old French (cf. French '-iste') and was later combined with 'arbitration' to form the modern coinage 'arbitrationist'.

Initially, the roots signified 'settlement by a judge-like figure' and 'one who does or supports'; over time these combined into a modern term meaning 'a person who practices or advocates arbitration', a relatively recent and somewhat rare formation in English.