erst ‧ while /ˈɜːstwaɪl $ ˈɜːrst-/ BrE AmE adjective [only before noun] formal
[ Date: 1500-1600 ; Origin: erst 'formerly' (11-19 centuries) (from Old English ærest 'earliest' ) + while ]
former or in the past:
She found herself ostracized by erstwhile friends.
his erstwhile allies