O.E.
sum "some," from P.Gmc.
*sumas (cf. O.S., O.Fris., O.H.G.
sum, O.N.
sumr, Goth.
sums), from PIE base
*sem- "one, as one" (cf. Skt.
samah "even, level, similar, identical;" Gk.
hamo-; see
same).
"The word has had greater currency in English than in the other Teutonic languages, in some of which it is now restricted to dialect use, or represented only by derivatives or compounds, as WFris. sommige, somlike, Du. sommige (also somtiids, sommijlen 'sometimes'), LG sömige (G. dial. summige)." [OED]
Meaning "remarkable" is attested from 1808, Amer.Eng. colloquial. A possessive form is attested from 1565, but always was rare. Most combination forms were in M.E.;
somehow is from 1664;
something once was very common as an adv. (cf.
something like).
Somebody in the sense of "important person" dates from 1566.
Somewhen is rare and since 19c. used almost exclusively in combination with more common compounds.
Get some "have sexual intercourse" is attested 1899 in a quote attributed to Abe Lincoln from c.1840.