1640s as a designation of one of the sides in the English Civil War (a partisan or supporter of Parliament as opposed to the King); meaning "one versed in parliamentary procedure" dates from 1834. See parliamentary + -ian.
variant of suffix -an (q.v.), with connective -i-. From Latin -ianus, in which the -i- originally was from the stem of the word being attached but later came to be felt as connective. In Middle English frequently it was -ien, via French.
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Definitions of parliamentarian
an expert in parliamentary rules and procedures;
an elected member of the British Parliament: a member of the House of Commons;