c. 1300, partiner, "a sharer or partaker in anything," altered from parcener (late 13c.), from Old French parçonier "partner, associate; joint owner, joint heir," from parçon "partition, division. portion, share, lot," from Latin partitionem (nominative partitio) "a sharing, partition, division, distribution" from past participle stem of partire "to part, divide" (from pars "a part, piece, a share," from PIE root *pere- (2) "to grant, allot").
The form in English has been influenced by part (n.). The word also may represent Old French part tenour "part holder." From late 14c. as "one who shares power or authority with another;" the commercial sense is by 1520s. Meaning "a husband or wife, one associated in marriage with another" is from 1749.