HCSB 1 Kings 2:37 On the day you do leave and cross the Kidron Valley, know for sure that you will certainly die.It's the same structure we find in Genesis, when God warns Adam and Eve against eating the forbidden fruit:
HCSB Gen2:17 ...on the day you eat from [the tree of the knowledge of good and evil], you will certainly die."On the day you do something, you will certainly die, or know you'll certainly die.
In the Hebrew, this "certainly die" is muwth-muwth - or "die-die". The meaning behind this repetition is that "dying, you will die".
So, in Genesis, on the day that Adam ate of the forbidden fruit, he was as good as dead, and on the day that Shimei violated his house-arrest by crossing the Kidron Valley on the way out of town, he should realize that he was as good as dead.
In both cases, the death is not immediate, but it is sealed. In Adam's case, it was immediately apparent because of physical changes that started taking place, including his realization that he was naked and needed covering. In Shimei's case, he probably thought no one would notice.