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]
late Old English agan, from earlier ongean (prep.) "toward; opposite, against, contrary to; in exchange for," as an adverb "in the opposite direction, back, to or toward a former place or position," from on "on" (see on (prep.) and compare a- (1)) + -gegn "against, toward," from Germanic root *gagina (source also of Old Norse gegn "straight, direct;" Danish igen "against;" Old Frisian jen, Old High German gegin, German gegen "against, toward," entgegen "against, in opposition to")
In Old English, eft (see eftsoons) was the main word for "again," but this often was strengthened by ongean, which became the principal word by 13c. Norse influence is responsible for the hard -g-. Differentiated from against (q.v.) 16c. in southern writers, again becoming an adverb only, and against taking over as preposition and conjunction, but again clung to all senses in northern and Scottish dialect (where against was not adopted). Of action, "in return," early 13c.; of action or fact, "once more," late 14c.
in geology, "leaning against," as one stratum of rock against another, both turned up at an angle, 1837, from Latin acclinis "leaning on or against," related to acclinare "to lean on or against," from assimilated form of ad "to, upon" (see ad-) + clinare "to bend" (from PIE *klein-, suffixed form of root *klei- "to lean").