The preposition between comes from the Old English word betweonum, meaning “among,” or “by turns.” If you are between jobs, you are not at your old job anymore and you haven’t yet found a new one — you are in the middle of your transition to something new, just as if you live between two feuding neighbors, you might feel caught in the middle. If you are stuck between a rock and a hard place, you are in some kind of trouble.