What does Matthew 5:35 mean?

Matthew 5:35 KJV
Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King.

Matthew 5:35 NKJV
“nor by the earth, for it is His footstool; nor by Jerusalem, for it is the city of the great King.

Matthew 5:35 MKJV
not by the earth, for it is the footstool of His feet; not by Jerusalem, because it is [the] city of the great King;

Matthew 5:35 KJV 2000
Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King.

Interlinear KJV

Nor /mete/ by /en/ the earth; /ge/ for /hoti/ it is /esti/ his /autos/ footstool /pous/: /hupopodion/ neither /mete/ by /eis/ Jerusalem; /Hierosoluma/ for /hoti/ it is /esti/ the city /polis/ of the great /megas/ King. /basileus/

Ripley 4 Gospels

Nor by the earth, for it is his footstool. See Isa 66:1. He who swears by the earth, does in reality appeal to God, and swear by him, as such an oath is a recognition of God himself, to whose dignity the earth is represented, in the Scripture, as an appendage.

The city of the great King. Ps 48:2. Swearing by Jerusalem was in effect swearing by the name of God, as Jerusalem owed its dignity and sanctity to the circumstance that it was the city which God had selected for his worship.