This is from the OED
arst, obs. form of erst adv.
erst, a. and adv. Forms: 1_3 ærest, ærost, ærst, 2_4 erest, 3_4 arst, (4 ? eroust, 5 erast, eerst), 3_7 earst, (3 earest), 4_6 erste, (6 ierst, 6, 7 yerst), 4_ erst.
[OE. ærest, superl. of ær (see ere); repr. OTeut. *airisto-, whence OHG. êrist (MHG. êrist, mod.G. erst), OS. êrist (Du. eerst).]
A. adj.
1. First in time or serial order. Obs.
2. absol. in advb. phrases. now (then) at erst: now (then) and not sooner; cf. Ger. erst dann. (App. through misunderstanding of this, Spenser has at erst = _as soon as possible, at once'.) on erst (only early ME.; also on alre erst): in the first place, at first.
B. adv.
1. Earliest, soonest, first in order of time.
2. In the first place, before something else is or was done; in preference to doing something else. (Sometimes pleonastically before ere.) Obs.
3. At first, as opposed to afterwards. Obs.
4. Sooner, earlier; before a specified time or event: esp. with negatives; not erst: not before, not till then. Obs.
5. Before the present time:
a. Referring to a remoter past: _Once upon a time', formerly, of old. arch. or poet.
b. Referring to a recent past: Not long ago, a little while since. Often in Spenser and Milton.
6. In comb. with adjs. and ppl. adjs.
1594 J. Dickenson Arisbas (1878) 33 Wherewith Flora had in plentie poudred the freshnesse of her earst-green hue.
1602 Carew Cornwall 100 a, The earst remembered Sir Ric_entertained at one time_the admirals of the English, Spanish, and Netherland fleets.
1740 Somerville Hobbinol (1749) 135 All but the stout And erst unconquer'd Hildebrand.
Hence erstly adv., in the first place.