Identical with the inflexion of the g. sg. is the -s surviving in certain adv. forms, as always, needs, nowadays, -wards, -ways; the use is exemplified by such OE. advs. as dæġes by day, sōðes in truth, truly, þances voluntarily. There were also OE. advs. compounded with tō TO and a genitive, as tōġeġnes against, tōmiddes amidst, by the side of which were synon. onġeġn AGAIN, onmiddan AMID; hence there arose in ME. mixed forms such as aʒeines, amiddes; and -(e)s became generalized, as in -WARDS, -WAYS. In once, twice, thrice, hence, thence, whence, since, the suffix has been otherwise spelt in order to avoid the suggestion of pronunc. with Z which is associated with -ns, viz. nz. In AGAINST, AMIDST, AMONGST there is a parasitic t.
In the disjunctive prons. hers, ours, theirs, yours (ME. hires, etc.) the -s is presumably analogical after his.
"-s." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (January 21, 2019). https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/s-10
"-s." The Concise Oxford Dictionary of English Etymology. . Retrieved January 21, 2019 from Encyclopedia.com: https://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/s-10