somehow adv.

Middle English som, "someone, somebody, a certain person; a certain indefinite portion of something, some part," from Old English sum "some, a, a certain one, something, a certain quantity; a certain indefinite number" (as in some say). This is from Proto-Germanic *sumaz (source also of Old Saxon, Old Frisian, Old High German sum, Old Norse sumr, Gothic sums), from a suffixed form of PIE root *sem- (1) "one; as one, together with."

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 .... [OED]

For substitution of -o- for -u-, see come. As a pronoun from c. 1100, "a certain quantity or number." A possessive form is attested from 1560s but remains rare. Use as a quasi-adverb before numerals began in Old English, originally "out of" (as in sum feowra "one of four").

The sense of "in some degree, to some extent" is American English, by 1745. The meaning "remarkable, quite a" is attested from 1808, American English colloquial.

Many combination forms (somewhat, sometime, somewhere) were in Middle English but often written as two words before 17-19c. Somewhen is rare and since 19c. used almost exclusively in combination with the more common compounds; somewho "someone" is attested from late 14c. but did not endure. Somewhy appeared occasionally in 19c. Scott (1816) has somegate "somewhere, in some way, somehow," and somekins or somskinnes "some kind of a" is recorded from c. 1200.

Get some "have sexual intercourse" is attested 1899 in an anecdote of Abe Lincoln from c. 1840.

Old English hu "how," from Proto-Germanic *hwo (source also of Old Saxon hwo, Old Frisian, Middle Dutch hu, Dutch hoe, German wie, Gothic hvaiwa "how"), an adverbial form from PIE root *kwo-, stem of relative and interrogative pronouns. Practically a doublet of why, differentiated in form and use.

How come? for "why?" is recorded from 1848 [Bartlett]. Emphatic phrase and how! is recorded from 1865. The formulation was common in book and article titles ("The National Debt, and How to Pay It"), but Pennsylvania writer Bayard Taylor, in whom it is first recorded, seems to have regarded it as a German or German-American expression.