Against
Middle English: from again + -s (adverbial genitive) + -t probably by association with superlatives (as in amongst ).
wiktionary
Formed from Middle English ayenes, agenes, againes(“in opposition to”), a southern variant of agen, or directly from again, either way with adverbial genitive singular ending -es; the parasitic -t was added circa 1350, probably by confusion with the superlative ending -est. Surface analysis again + -st(excrescent ending).
etymonline
against (prep.)
12c., agenes "in opposition to, adverse, hostile; in an opposite direction or position, in contact with, in front of, so as to meet," originally a southern variant of agan (prep.) "again" (see again), with adverbial genitive. The unetymological -t turned up mid-14c. and was standard by early 16c., perhaps from influence of superlatives (see amidst). Use as a conjunction, "against the time that," hence "before," is now archaic or obsolete.
