'arbitrariness' originates from the adjective 'arbitrary' combined with the nominalizing suffix '-ness'. 'Arbitrary' ultimately comes from Latin 'arbiter' (judge, witness) and the derived 'arbitrarius' (depending on the decision of an arbiter).

'Arbiter' in Latin led to Late Latin 'arbitrarius' meaning 'depending on the judgment of an arbiter'; this passed into Old French as words like 'arbitraire' and into Middle English as 'arbitrarie' or 'arbitrary', from which English formed the noun 'arbitrariness' with '-ness'.

Initially related to the role or decision of a judge or arbiter ('one who judges'), the sense shifted to denote actions determined by a single will or discretion; over time it further generalized to mean 'decision or behavior based on whim or caprice'—the current sense of 'arbitrariness'.