A kingdom is made up of 2 words king + dom(ain). The domain over which Jesus plus his 144,000 co-rulers will reign is the earth. It will result in ''Thy(God's) will be(ing) done in earth''.
Made up by your mates at Watchtower HQ. To top it all they tell their flock that the 144,000 are selected Watchtower type JWs. The same crowd who advocate shunning for refusing to go along with an anti-Christ doctrine.
Talk about delusions of grandeur. Here is an example in spades.
