From Old English 'aerest' (earliest) + 'while' (time) — 'erst' traces to PIE *h2eyer- (day, morning).
Former; of a previous time.
From 'erst' (formerly, once) + '-while' (a period of time). 'Erst' comes from Old English 'ǣrest' (earliest, first), the superlative of 'ǣr' (before, early, formerly), from Proto-Germanic *airiz (earlier, before), from PIE *h₂eyer- (day, morning, early part of day). The same PIE root produced German 'erst' (first, only then), Dutch 'eerst' (first), and connects distantly to Latin 'aurora' (dawn) and Sanskrit
The word is a purely Germanic compound that has maintained a literary, formal quality throughout its history.