Old English cyningdom; see king (n.) + -dom. Cognate with Old Saxon kuningdom, Middle Dutch koninghdom, Old Norse konungdomr. The usual Old English word was cynedom; Middle English also had kingrick (for second element, see the first element in Reichstag). Meaning "one of the realms of nature" is from 1690s.
Kingdom-come (n.) "the next world, the hereafter" (1785), originally slang, is from the Lord's Prayer, where it is an archaic simple present subjunctive ("may Thy kingdom come") in reference to the spiritual reign of God or Christ.