Words: erst, erstwhile [MW]

Function: adverb
Etymology: Middle English erest earliest, formerly, from Old English [AE]rest, superlative of [AE]r early -- more at ERE
archaic : ERSTWHILE

Function: adverb
: in the past : FORMERLY <cultures, erstwhile unknown to each other -- Robert Plank>
Function: adjective
: FORMER, PREVIOUS <erstwhile enemies>