The Greek word from which “Lord” is derived in this Ephesians 4:5 is kurios.  Commentator William Barclay describes the two uses for this single word in ordinary Greek:

Master. in contradistinction to servant or slave.

It was also the regular designation for the Roman emperor.

He adds this insight, “Christians are joined together because they are all in the possession and in the service of the one master and king.”