The original of this passage is in Mark's Gospel, simply because it is the strong consensus of New Testament scholars that Mark was written first and was the main source used by the authors of the other two synoptic gospels. Thus, the most reliable way to establish the original meaning of this passage seems to be to look in Mark. The term 'kingdom of God' can tend to hide meaning and, in Mark as Story, Rhoads, Dewey and Michie suggest a better translation to be 'rule of God', the Greek being capable of either meaning but 'rule of God', although less regal, a more straightforward reading in the English language.
In Mark 9:1, Jesus is prophesying his second coming, a prophecy he repeats in more dramatic detail in chapter 13. Mark has the ability to create relationships between events by the use of several rhetorical literary devices, including intercalations (literary sandwiches), chiastic structures, parallel structures and proximity. In this case, the prophecy in Mark 9:1 is linked, as pair X, in a proposed parallel structure of Mark, a literary relationship that demonstrates authorial intent. Mark 9:1 also follows seamlessly after a prophecy of the crucifixion in verse 8:31, with just a short intervening passage that hints of Peter's feelings of shame after the three denials.
Mark 8:31: And he began to teach them, that the Son of man must suffer many things, and be rejected of the elders, and of the chief priests, and scribes, and be killed, and after three days rise again.
Mark 9:1 (KJV): And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.
Mark chapter 13 opens with a graphic description of the events of the First Roman-Jewish War and the civil war that erupted within Jerusalem even as it was being besieged. Then they are told that Jesus will come with great power and glory before his present generation had passed:
Mark 13:26-30 And then shall they see the Son of man coming in the clouds with great power and glory. And then shall he send his angels, and shall gather together his elect from the four winds, from the uttermost part of the earth to the uttermost part of heaven. Now learn a parable of the fig tree; When her branch is yet tender, and putteth forth leaves, ye know that summer is near: So ye in like manner, when ye shall see these things come to pass, know that it is nigh, even at the doors. Verily I say unto you, that this generation shall not pass, till all these things be done.
Jesus did not literally prophesy his return on clouds within the lifetimes of some to whom he spoke, because had he done so, he would have been wrong. But the author of Mark had seen the tumultuous events of 70 CE and believed they were a portent of the second coming. This is what was meant by "will not taste death until they see the kingdom of God."
By the time the Gospels of Matthew and Luke were written, it was becoming clear that Jesus' generation had passed away without having seen Jesus return to establish the kingdom of God, so in copying Mark chapter 13, they emphasised that no one knew when Jesus would return, but they did copy the less obvious prophecy in Mark 9:1.