A. In adjs., ME. -er(e), -re, OE. -ra m., -re fem., n. repr. two Gmc. suffixes, (i) *-izan- (OS., OHG. -iro, ON, -ri, Goth. -iza, accompanied by mutation) and (ii) *-ōzan- (OHG. -ōro, ON. -ari, Goth, -ōza), which were formed on the adv. suffixes *-iz, *-ōz (see below). A few OE. compars. show mutation, as strengra (f. strang strong); traces have remained in better :- *batizan- and elder :- *alðizan-; worse and less contain the suffix *-izan- in a disguised form.
B. In advs., OE. -or = OS., OHG. -ōr, Goth. -ōz :- Gmc. *-ōz, beside which there was *-iz (corr. to L. -is, as in magis more, nimis too much), repr. by ON. -r (with mutation) and Goth. -is (e.g. hauhis higher), and by the mutation in OE. compars. like leng longer :- *laŋgiz, bet better:- *batiz, which were superseded in ME. by regular forms in -er. Cf. -EST.
"-er." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (February 22, 2018). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/er-7
"-er." The Concise Oxford Dictionary of English Etymology. . Retrieved February 22, 2018 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/er-7