someday 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 dæg "period during which the sun is above the horizon," also "lifetime, definite time of existence," from Proto-Germanic *dages- "day" (source also of Old Saxon, Middle Dutch, Dutch dag, Old Frisian di, dei, Old High German tag, German Tag, Old Norse dagr, Gothic dags), according to Watkins, from PIE root *agh- "a day."  He adds that the Germanic initial d- is "of obscure origin." But Boutkan says it is from PIE root *dhegh- "to burn" (see fever). Not considered to be related to Latin dies (which is from PIE root *dyeu- "to shine").

Meaning originally, in English, "the daylight hours;" it expanded to mean "the 24-hour period" in late Anglo-Saxon times. The day formerly began at sunset, hence Old English Wodnesniht was what we would call "Tuesday night." Names of the weekdays were not regularly capitalized in English until 17c.

From late 12c. as "a time period as distinguished from other time periods." Day-by-day "daily" is from late 14c.; all day "all the time" is from late 14c.  Day off "day away from work" is attested from 1883; day-tripper first recorded 1897. The days in nowadays, etc. is a relic of the Old English and Middle English use of the adverbial genitive.

All in a day's work "something unusual taken as routine" is by 1820. The nostalgic those were the days is attested by 1907. That'll be the day, expressing mild doubt following some boast or claim, is by 1941. To call it a day "stop working" is by 1919; earlier call it a half-day (1838). One of these days "at some day in the near future" is from late 15c. One of those days "a day of misfortune" is by 1936.