Commoner

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Middle English (denoting a citizen or burgess): from medieval Latin communarius, from communa, communia ‘community’, based on Latin communis (see common).

wiktionary

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common +‎  -er(“comparative suffix”)

From Middle English comoner, comyner, cumuner, equivalent to common +‎ -er.

etymonline

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commoner (n.)

late 14c. (mid-14c. in Anglo-French), "one of the common people, a member of the third estate," agent noun from common (v.) "participate in common, associate or have dealings with" (mid-14c.), from common (adj.). From mid-15c. as "member of the House of Commons."