Old English betweox "between, among, amidst, meanwhile," from bi- "by" (see be-) + tweox "for two," from Proto-Germanic *twa "two" + *-isk "-ish." With parasitic -t that first appeared in Old English and became general after c.1500.
Undecided, midway between two alternatives, neither here nor there. For example, I'm betwixt and between canceling my trip entirely or just postponing it, or Jane is betwixt and between about accepting the offer. The adverb betwixt, originally meaning “by two,” is seldom heard except in this expression, first recorded in 1832.