means A person who shares an activity, business, relationship, or undertaking with you, with joint stakes in the outcome.

from From Old French 'parçonier' (a sharer or co-heir), rooted in Latin 'partitionem,' a sharing or dividing — the same 'part' you split when you split anything. English speakers reshaped the word toward 'part,' so a partner became, fittingly, someone who holds a part of what's yours.