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]