Why is the ninth month called September?
c.1300, 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" (see partition (n.)). Form in English influenced by part (n.). The word also may represent Old French part tenour "part holder."
1610s, transitive, "to make a partner," from partner (n.). Intransitive sense from 1961. Related: Partnered; partnering.