"between, betwixt," late 14c., atwix, atwixen, from a- (1) + Middle English twix, twixt "among" (see betwixt). The unetymological -t begins by mid-15c. (compare amidst).
Entries linking to atwixt
a- 1
prefix or inseparable particle, a conglomerate of various Germanic and Latin elements.
In words derived from Old English, it commonly represents Old English an "on, in, into" (see on (prep.)), as in alive, above, asleep, aback, abroad, afoot, ashore, ahead, abed, aside, obsolete arank "in rank and file," etc., forming adjectives and adverbs from nouns, with the notion "in, at; engaged in." In this use it is identical to a (2).
It also can represent Middle English of (prep.) "off, from," as in anew, afresh, akin, abreast. Or it can be a reduced form of the Old English past participle prefix ge-, as in aware.
Or it can be the Old English intensive a-, originally ar- (cognate with German er- and probably implying originally "motion away from"), as in abide, arise, awake, ashamed, marking a verb as momentary, a single event. Such words sometimes were refashioned in early modern English as though the prefix were Latin (accursed, allay, affright are examples).
In words from Romanic languages, often it represents reduced forms of Latin ad "to, toward; for" (see ad-), or ab "from, away, off" (see ab-); both of which by about 7c. had been reduced to a in the ancestor of Old French. In a few cases it represents Latin ex.
[I]t naturally happened that all these a- prefixes were at length confusedly lumped together in idea, and the resultant a- looked upon as vaguely intensive, rhetorical, euphonic, or even archaic, and wholly otiose. [OED]
betwixt prep., adv.
Middle English bitwixe, from Old English betweox "between, in the space that separates, among, amidst, meanwhile," from bi- "by" (see by) + tweox "for two," from Proto-Germanic *twa "two" (from PIE root *dwo- "two") + *-isk "-ish."
With unetymological -t that appeared in Old English and became general after c. 1500. Compare amidst. Betwixen also was a variant in Old and Middle English. Middle English also had twix (prep., adv.) "among; in the meantime." Now mostly in the colloquial intensive expression betwixt and between.
amidst prep.
a variant of amid (q.v.) with adverbial genitive -s and unetymological -t. Amidde became amyddes (13c.) and acquired the -t from mid-15c., probably by association with superlatives in -st; the pattern also yielded amongst, against, betwixt, whilst, also archaic alongst (13c.-17c.).
There is a tendency to use amidst more distributively than amid, e.g. of things scattered about, or a thing moving, in the midst of others. [OED]